>If you can post the query maybe? things such as LIKE and other things can cause issues too.

Here the query, if I select around 2-3 unions it works, but once it gets more it seems to fail. So the query in itself is OK, just when there are too many unions it will break, just as if it would cut off the string somewhere in the middle.
I think it would be better to build up the where clause with OR conditions and join with the table that is scanned to get the conditions.

But I would have thought this would be working as well.

		SELECT 
			MFB699.MFBWNr,
			MIN(MFB699.SrtCode) AS SrtCode,  /* Not necessary ? */
		 	MFB699.MFBWOms, 
			MFB681.MFBPNr, 
			MFB681.MFBProc, 
			MFB681.MFBPrR,

			MFB690.MFBPDVV,
			MFB690.MFBPOMS,
			MFB690.MFBPWIN,
			MFB690.MFBPWIND,
			MFB690.MFBKNR AS MFBKNr_Start,

			MFB691.MFBPNrV,
			MFB691.MFBVNr,
			MFB691.MFBKNr,
			MFB691.MFBPJK,
			MFB691.MFBPJa,
			IFNULL(MFB693_Ja.MFBAJN,'') AS MFBAJN_Ja,
			MFB691.MFBPNK,
			MFB691.MFBPNa, 
			IFNULL(MFB693_Na.MFBAJN,'') AS MFBAJN_Na,

			MFB692.MFBVOms, 
			MFB692.MFBFuWo,
			MFB692.MFBFunNr,
			MFB692.MFBVStJ,
			MFB692.MFBVStJT,
			MFB692.MFBVStN,
			MFB692.MFBVStNT,
			MFB692.MFBVOPER,
			MFB692.MFBVW,
			
			CAST(1 AS Decimal(1,0)) AS Status,
			CAST("58699D49-DBE4-4D2D-B476-E51C76710194" AS Character(36)) AS RecordKey,
			CAST(0 AS Decimal(1,0)) AS Interactie,
			CAST("" AS Character(36)) AS RecordKeyIA,
			CAST("" AS Character(240)) AS Omschr,
			IFNULL(Memofield.MeMemo,"") AS Omschr2,
			CAST(0 AS Decimal(1,0)) AS Result,               /* Functie Result: 1 = Ja, 2 = Nee */
			CAST(0 AS Decimal(1,0)) AS Aktie,                /* Aktie (only if MFBPJa or MFBPNa): 1 = Ja, 2 = Nee */
			CAST('0000000000' AS Character(10)) AS AktieNr,    /* Aktie nummer */
			CAST('0000000000' AS Character(10)) AS AktieKNr, /* next Knooppuntnummer nummer */
			IFNULL(Memofield.MeMemo,"") AS Links
			
			FROM MFB699 
			STRAIGHT_JOIN MFB681 ON MFB681.MFBWNr = MFB699.MFBWNr 
			STRAIGHT_JOIN MFB690 ON MFB690.MFBPNr = MFB681.MFBPNr
			STRAIGHT_JOIN MFB691 ON MFB691.MFBPNr = MFB681.MFBPNr
				AND MFB691.MFBPNrV = MFB690.MFBPNrV
			STRAIGHT_JOIN MFB692 ON MFB692.MFBVNr = MFB691.MFBVNr
			LEFT JOIN MFB698 ON MFB698.MFBPNr = MFB681.MFBPNr
				AND MFB698.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocol ON MFBSetProtocol.DelFlag = 0
				AND MFBSetProtocol.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocol.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocolPat ON MFBSetProtocolPat.DelFlag = 0
				AND MFBSetProtocolPat.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocolPat.msPaKey = ?pcPaKey
			
			LEFT JOIN MFB693 MFB693_Ja ON MFB693_Ja.MFBANr = MFB691.MFBPJa
			LEFT JOIN MFB693 MFB693_Na ON MFB693_Na.MFBANr = MFB691.MFBPNa
			
			LEFT JOIN Memofield ON 1=2
			WHERE 1=1
			  AND MFB690.MFBPWIN = "N"
			   AND MFBSetProtocol.DelFlag = 0  AND ( IFNULL(MFBSetProtocolPat.msActive, MFBSetProtocol.msActive) = 1 ) 
			   AND MFB681.MFBPrR = ?pnProcess  AND MFB698.MFBLBLNR = 5 
			   AND (
 (MFB699.CodeNV = 00101214 AND MFB699.SrtCode = 40)
  OR  (MFB699.CodeNV = 00047481 AND MFB699.SrtCode = 45)
  OR  (MFB699.CodeNV = 01757040 AND MFB699.SrtCode = 50))
			GROUP BY 1,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34
					 UNION ALL 
		SELECT 
			MFB699.MFBWNr,
			MIN(MFB699.SrtCode) AS SrtCode,  /* Not necessary ? */
		 	MFB699.MFBWOms, 
			MFB681.MFBPNr, 
			MFB681.MFBProc, 
			MFB681.MFBPrR,

			MFB690.MFBPDVV,
			MFB690.MFBPOMS,
			MFB690.MFBPWIN,
			MFB690.MFBPWIND,
			MFB690.MFBKNR AS MFBKNr_Start,

			MFB691.MFBPNrV,
			MFB691.MFBVNr,
			MFB691.MFBKNr,
			MFB691.MFBPJK,
			MFB691.MFBPJa,
			IFNULL(MFB693_Ja.MFBAJN,'') AS MFBAJN_Ja,
			MFB691.MFBPNK,
			MFB691.MFBPNa, 
			IFNULL(MFB693_Na.MFBAJN,'') AS MFBAJN_Na,

			MFB692.MFBVOms, 
			MFB692.MFBFuWo,
			MFB692.MFBFunNr,
			MFB692.MFBVStJ,
			MFB692.MFBVStJT,
			MFB692.MFBVStN,
			MFB692.MFBVStNT,
			MFB692.MFBVOPER,
			MFB692.MFBVW,
			
			CAST(1 AS Decimal(1,0)) AS Status,
			CAST("D1758CF0-1CDA-481F-AF84-6C5D49A6323D" AS Character(36)) AS RecordKey,
			CAST(0 AS Decimal(1,0)) AS Interactie,
			CAST("" AS Character(36)) AS RecordKeyIA,
			CAST("" AS Character(240)) AS Omschr,
			IFNULL(Memofield.MeMemo,"") AS Omschr2,
			CAST(0 AS Decimal(1,0)) AS Result,               /* Functie Result: 1 = Ja, 2 = Nee */
			CAST(0 AS Decimal(1,0)) AS Aktie,                /* Aktie (only if MFBPJa or MFBPNa): 1 = Ja, 2 = Nee */
			CAST('0000000000' AS Character(10)) AS AktieNr,    /* Aktie nummer */
			CAST('0000000000' AS Character(10)) AS AktieKNr, /* next Knooppuntnummer nummer */
			IFNULL(Memofield.MeMemo,"") AS Links
			
			FROM MFB699 
			STRAIGHT_JOIN MFB681 ON MFB681.MFBWNr = MFB699.MFBWNr 
			STRAIGHT_JOIN MFB690 ON MFB690.MFBPNr = MFB681.MFBPNr
			STRAIGHT_JOIN MFB691 ON MFB691.MFBPNr = MFB681.MFBPNr
				AND MFB691.MFBPNrV = MFB690.MFBPNrV
			STRAIGHT_JOIN MFB692 ON MFB692.MFBVNr = MFB691.MFBVNr
			LEFT JOIN MFB698 ON MFB698.MFBPNr = MFB681.MFBPNr
				AND MFB698.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocol ON MFBSetProtocol.DelFlag = 0
				AND MFBSetProtocol.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocol.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocolPat ON MFBSetProtocolPat.DelFlag = 0
				AND MFBSetProtocolPat.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocolPat.msPaKey = ?pcPaKey
			
			LEFT JOIN MFB693 MFB693_Ja ON MFB693_Ja.MFBANr = MFB691.MFBPJa
			LEFT JOIN MFB693 MFB693_Na ON MFB693_Na.MFBANr = MFB691.MFBPNa
			
			LEFT JOIN Memofield ON 1=2
			WHERE 1=1
			  AND MFB690.MFBPWIN = "N"
			   AND MFBSetProtocol.DelFlag = 0  AND ( IFNULL(MFBSetProtocolPat.msActive, MFBSetProtocol.msActive) = 1 ) 
			   AND MFB681.MFBPrR = ?pnProcess  AND MFB698.MFBLBLNR = 5 
			   AND (
 (MFB699.CodeNV = 00093327 AND MFB699.SrtCode = 40)
  OR  (MFB699.CodeNV = 00039551 AND MFB699.SrtCode = 45)
  OR  (MFB699.CodeNV = 01791583 AND MFB699.SrtCode = 50))
			GROUP BY 1,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34
					 UNION ALL 
		SELECT 
			MFB699.MFBWNr,
			MIN(MFB699.SrtCode) AS SrtCode,  /* Not necessary ? */
		 	MFB699.MFBWOms, 
			MFB681.MFBPNr, 
			MFB681.MFBProc, 
			MFB681.MFBPrR,

			MFB690.MFBPDVV,
			MFB690.MFBPOMS,
			MFB690.MFBPWIN,
			MFB690.MFBPWIND,
			MFB690.MFBKNR AS MFBKNr_Start,

			MFB691.MFBPNrV,
			MFB691.MFBVNr,
			MFB691.MFBKNr,
			MFB691.MFBPJK,
			MFB691.MFBPJa,
			IFNULL(MFB693_Ja.MFBAJN,'') AS MFBAJN_Ja,
			MFB691.MFBPNK,
			MFB691.MFBPNa, 
			IFNULL(MFB693_Na.MFBAJN,'') AS MFBAJN_Na,

			MFB692.MFBVOms, 
			MFB692.MFBFuWo,
			MFB692.MFBFunNr,
			MFB692.MFBVStJ,
			MFB692.MFBVStJT,
			MFB692.MFBVStN,
			MFB692.MFBVStNT,
			MFB692.MFBVOPER,
			MFB692.MFBVW,
			
			CAST(2 AS Decimal(1,0)) AS Status,
			CAST("900D38E4-E3A0-4921-9701-66B8987435C9" AS Character(36)) AS RecordKey,
			CAST(0 AS Decimal(1,0)) AS Interactie,
			CAST("" AS Character(36)) AS RecordKeyIA,
			CAST("" AS Character(240)) AS Omschr,
			IFNULL(Memofield.MeMemo,"") AS Omschr2,
			CAST(0 AS Decimal(1,0)) AS Result,               /* Functie Result: 1 = Ja, 2 = Nee */
			CAST(0 AS Decimal(1,0)) AS Aktie,                /* Aktie (only if MFBPJa or MFBPNa): 1 = Ja, 2 = Nee */
			CAST('0000000000' AS Character(10)) AS AktieNr,    /* Aktie nummer */
			CAST('0000000000' AS Character(10)) AS AktieKNr, /* next Knooppuntnummer nummer */
			IFNULL(Memofield.MeMemo,"") AS Links
			
			FROM MFB699 
			STRAIGHT_JOIN MFB681 ON MFB681.MFBWNr = MFB699.MFBWNr 
			STRAIGHT_JOIN MFB690 ON MFB690.MFBPNr = MFB681.MFBPNr
			STRAIGHT_JOIN MFB691 ON MFB691.MFBPNr = MFB681.MFBPNr
				AND MFB691.MFBPNrV = MFB690.MFBPNrV
			STRAIGHT_JOIN MFB692 ON MFB692.MFBVNr = MFB691.MFBVNr
			LEFT JOIN MFB698 ON MFB698.MFBPNr = MFB681.MFBPNr
				AND MFB698.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocol ON MFBSetProtocol.DelFlag = 0
				AND MFBSetProtocol.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocol.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocolPat ON MFBSetProtocolPat.DelFlag = 0
				AND MFBSetProtocolPat.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocolPat.msPaKey = ?pcPaKey
			
			LEFT JOIN MFB693 MFB693_Ja ON MFB693_Ja.MFBANr = MFB691.MFBPJa
			LEFT JOIN MFB693 MFB693_Na ON MFB693_Na.MFBANr = MFB691.MFBPNa
			
			LEFT JOIN Memofield ON 1=2
			WHERE 1=1
			  AND MFB690.MFBPWIN = "N"
			   AND MFBSetProtocol.DelFlag = 0  AND ( IFNULL(MFBSetProtocolPat.msActive, MFBSetProtocol.msActive) = 1 ) 
			   AND MFB681.MFBPrR = ?pnProcess  AND MFB698.MFBLBLNR = 5 
			   AND (
 (MFB699.CodeNV = 00101575 AND MFB699.SrtCode = 40)
  OR  (MFB699.CodeNV = 00061077 AND MFB699.SrtCode = 45)
  OR  (MFB699.CodeNV = 01979566 AND MFB699.SrtCode = 50))
			GROUP BY 1,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34
					 UNION ALL 
		SELECT 
			MFB699.MFBWNr,
			MIN(MFB699.SrtCode) AS SrtCode,  /* Not necessary ? */
		 	MFB699.MFBWOms, 
			MFB681.MFBPNr, 
			MFB681.MFBProc, 
			MFB681.MFBPrR,

			MFB690.MFBPDVV,
			MFB690.MFBPOMS,
			MFB690.MFBPWIN,
			MFB690.MFBPWIND,
			MFB690.MFBKNR AS MFBKNr_Start,

			MFB691.MFBPNrV,
			MFB691.MFBVNr,
			MFB691.MFBKNr,
			MFB691.MFBPJK,
			MFB691.MFBPJa,
			IFNULL(MFB693_Ja.MFBAJN,'') AS MFBAJN_Ja,
			MFB691.MFBPNK,
			MFB691.MFBPNa, 
			IFNULL(MFB693_Na.MFBAJN,'') AS MFBAJN_Na,

			MFB692.MFBVOms, 
			MFB692.MFBFuWo,
			MFB692.MFBFunNr,
			MFB692.MFBVStJ,
			MFB692.MFBVStJT,
			MFB692.MFBVStN,
			MFB692.MFBVStNT,
			MFB692.MFBVOPER,
			MFB692.MFBVW,
			
			CAST(2 AS Decimal(1,0)) AS Status,
			CAST("B1D8C693-0F09-4669-BC1E-570A31111F0C" AS Character(36)) AS RecordKey,
			CAST(0 AS Decimal(1,0)) AS Interactie,
			CAST("" AS Character(36)) AS RecordKeyIA,
			CAST("" AS Character(240)) AS Omschr,
			IFNULL(Memofield.MeMemo,"") AS Omschr2,
			CAST(0 AS Decimal(1,0)) AS Result,               /* Functie Result: 1 = Ja, 2 = Nee */
			CAST(0 AS Decimal(1,0)) AS Aktie,                /* Aktie (only if MFBPJa or MFBPNa): 1 = Ja, 2 = Nee */
			CAST('0000000000' AS Character(10)) AS AktieNr,    /* Aktie nummer */
			CAST('0000000000' AS Character(10)) AS AktieKNr, /* next Knooppuntnummer nummer */
			IFNULL(Memofield.MeMemo,"") AS Links
			
			FROM MFB699 
			STRAIGHT_JOIN MFB681 ON MFB681.MFBWNr = MFB699.MFBWNr 
			STRAIGHT_JOIN MFB690 ON MFB690.MFBPNr = MFB681.MFBPNr
			STRAIGHT_JOIN MFB691 ON MFB691.MFBPNr = MFB681.MFBPNr
				AND MFB691.MFBPNrV = MFB690.MFBPNrV
			STRAIGHT_JOIN MFB692 ON MFB692.MFBVNr = MFB691.MFBVNr
			LEFT JOIN MFB698 ON MFB698.MFBPNr = MFB681.MFBPNr
				AND MFB698.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocol ON MFBSetProtocol.DelFlag = 0
				AND MFBSetProtocol.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocol.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocolPat ON MFBSetProtocolPat.DelFlag = 0
				AND MFBSetProtocolPat.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocolPat.msPaKey = ?pcPaKey
			
			LEFT JOIN MFB693 MFB693_Ja ON MFB693_Ja.MFBANr = MFB691.MFBPJa
			LEFT JOIN MFB693 MFB693_Na ON MFB693_Na.MFBANr = MFB691.MFBPNa
			
			LEFT JOIN Memofield ON 1=2
			WHERE 1=1
			  AND MFB690.MFBPWIN = "N"
			   AND MFBSetProtocol.DelFlag = 0  AND ( IFNULL(MFBSetProtocolPat.msActive, MFBSetProtocol.msActive) = 1 ) 
			   AND MFB681.MFBPrR = ?pnProcess  AND MFB698.MFBLBLNR = 5 
			   AND (
 (MFB699.CodeNV = 00101575 AND MFB699.SrtCode = 40)
  OR  (MFB699.CodeNV = 00061077 AND MFB699.SrtCode = 45)
  OR  (MFB699.CodeNV = 01979566 AND MFB699.SrtCode = 50))
			GROUP BY 1,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34
					 UNION ALL 
		SELECT 
			MFB699.MFBWNr,
			MIN(MFB699.SrtCode) AS SrtCode,  /* Not necessary ? */
		 	MFB699.MFBWOms, 
			MFB681.MFBPNr, 
			MFB681.MFBProc, 
			MFB681.MFBPrR,

			MFB690.MFBPDVV,
			MFB690.MFBPOMS,
			MFB690.MFBPWIN,
			MFB690.MFBPWIND,
			MFB690.MFBKNR AS MFBKNr_Start,

			MFB691.MFBPNrV,
			MFB691.MFBVNr,
			MFB691.MFBKNr,
			MFB691.MFBPJK,
			MFB691.MFBPJa,
			IFNULL(MFB693_Ja.MFBAJN,'') AS MFBAJN_Ja,
			MFB691.MFBPNK,
			MFB691.MFBPNa, 
			IFNULL(MFB693_Na.MFBAJN,'') AS MFBAJN_Na,

			MFB692.MFBVOms, 
			MFB692.MFBFuWo,
			MFB692.MFBFunNr,
			MFB692.MFBVStJ,
			MFB692.MFBVStJT,
			MFB692.MFBVStN,
			MFB692.MFBVStNT,
			MFB692.MFBVOPER,
			MFB692.MFBVW,
			
			CAST(0 AS Decimal(1,0)) AS Status,
			CAST("BB78EA7E-2575-4936-910F-E18AE26848EC" AS Character(36)) AS RecordKey,
			CAST(0 AS Decimal(1,0)) AS Interactie,
			CAST("" AS Character(36)) AS RecordKeyIA,
			CAST("" AS Character(240)) AS Omschr,
			IFNULL(Memofield.MeMemo,"") AS Omschr2,
			CAST(0 AS Decimal(1,0)) AS Result,               /* Functie Result: 1 = Ja, 2 = Nee */
			CAST(0 AS Decimal(1,0)) AS Aktie,                /* Aktie (only if MFBPJa or MFBPNa): 1 = Ja, 2 = Nee */
			CAST('0000000000' AS Character(10)) AS AktieNr,    /* Aktie nummer */
			CAST('0000000000' AS Character(10)) AS AktieKNr, /* next Knooppuntnummer nummer */
			IFNULL(Memofield.MeMemo,"") AS Links
			
			FROM MFB699 
			STRAIGHT_JOIN MFB681 ON MFB681.MFBWNr = MFB699.MFBWNr 
			STRAIGHT_JOIN MFB690 ON MFB690.MFBPNr = MFB681.MFBPNr
			STRAIGHT_JOIN MFB691 ON MFB691.MFBPNr = MFB681.MFBPNr
				AND MFB691.MFBPNrV = MFB690.MFBPNrV
			STRAIGHT_JOIN MFB692 ON MFB692.MFBVNr = MFB691.MFBVNr
			LEFT JOIN MFB698 ON MFB698.MFBPNr = MFB681.MFBPNr
				AND MFB698.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocol ON MFBSetProtocol.DelFlag = 0
				AND MFBSetProtocol.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocol.MFBPNrV = MFB690.MFBPNrV

			LEFT JOIN MFBSetProtocolPat ON MFBSetProtocolPat.DelFlag = 0
				AND MFBSetProtocolPat.MFBPNr = MFB690.MFBPNr
				AND MFBSetProtocolPat.msPaKey = ?pcPaKey
			
			LEFT JOIN MFB693 MFB693_Ja ON MFB693_Ja.MFBANr = MFB691.MFBPJa
			LEFT JOIN MFB693 MFB693_Na ON MFB693_Na.MFBANr = MFB691.MFBPNa
			
			LEFT JOIN Memofield ON 1=2
			WHERE 1=1
			  AND MFB690.MFBPWIN = "N"
			   AND MFBSetProtocol.DelFlag = 0  AND ( IFNULL(MFBSetProtocolPat.msActive, MFBSetProtocol.msActive) = 1 ) 
			   AND MFB681.MFBPrR = ?pnProcess  AND MFB698.MFBLBLNR = 5 
			   AND (
 (MFB699.CodeNV = 00002194 AND MFB699.SrtCode = 40)
  OR  (MFB699.CodeNV = 00067903 AND MFB699.SrtCode = 45)
  OR  (MFB699.CodeNV = 02351595 AND MFB699.SrtCode = 50))
			GROUP BY 1,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34
				ORDER BY Status, MFBPNr, MFBPNrV, MFBKNr