>I do a Scan right after the filter and then reset the filter (SET FILTER TO) right after the scan. The fatal error message is reported to occur on the SCAN statement. I'll avoid using variables in filters as much as possible now, but do you think it was the cuplrit in this case?

If that's what you're doing, I'd strongly suggest using

SCAN FOR (filtercondition)

instead of

SET FILTER TO (filtercondition)
SCAN
ENDSCAN
SET FILTER TO

You should see much better performance with a Rushmorized FOR clause than with a SET FILTER command.

HTH,