>To all who have responded to this problem - thanks a lot and I have now resolved it.
>
>I stepwise took tables out of the dud forms' DEs, till the forms ran without error, then I reinstated them. It seems that the DE may have been holding some out-of-date tags but this wasn't apparent when you looked at them (i.e. same name but changed slightly since the form was created - but the form still "thought" in terms of the old tag). Putting back the tables with their NEW index format seems to have fixed it. Go figure!

Here's the simple builder-like .prg I use for checking DSes:

=ASELOBJ(aa,2)
FOR EACH oCur IN aa[1].objects
	IF oCur.BaseClass='Cursor'
* give them proper names, cursor1... cursor32 don't mean a thing in PEM window:
		oCur.Name='cur'+PROPER(oCur.Alias)
		IF !EMPTY(oCur.Database)
				OPEN DATABASE (oCur.Database)
		ENDIF
		IF !USED(oCur.alias)
			SELECT 0
			USE (oCur.CursorSource) again ALIAS (oCur.alias)
		ENDIF
		SELECT (oCur.alias)
		IF !EMPTY(oCur.Filter)
			IF TYPE(ocur.Filter)#"L"
				=messagebox('bad filter in '+ocur.Name)
			ENDIF
		endif
		IF !EMPTY(oCur.order)
			IF TAGNO(ocur.Order)=0
				=messagebox('bad order in '+ocur.Name)
			ENDIF
		endif
	ENDIF
endfor