To make it simpler:
SELECT IIF(empty(dtcompl),"''","'" + DTOC(dtcompl) + "'") from mevmast where citat LIKE 'BA83311%'

results in '0

SELECT IIF(empty(dtdisp),"''","'" + DTOC(dtdisp) + "'") from mevmast where citat LIKE 'BA83311%'

results in '10/26/90'

Both fields are of the date type, length 8, and contain a date value. What could cause this?