When investigating a customer's database to see if it will easily migrate to V8.5 from V6.52 I am getting failures of the migration when it is Reorganising Application Documents.

For some reason even simple DQLs are failing with this error message (I have typed one out below).

for DelNoteFooter ;
modify records
DelPrinted := yes.

The table and the field exist, the field is a yes/no field defaulting to no.

Can anyone tell me what might be causing this?

Try to do a backup/restore of the 6.x application before migrating it. 

The problem in DFW is that everything is stored in Binary code which are loaded directly into memory. So if its corrupt it can cause all kinds of problems. GPFs does not need to be caused by where they are "displayed" it can be caused by something preceding it and then the corrupt state will cause the GPF when you load the next or a later document etc.

Make sure that the application is consistent and functioning well before migrating will be he the best bet.

A Backup/Restore had been done but the problems remained.

The system is running successfully under 6.52 with no operational problems.

I think that there must be a corruption somewhere not just because of the Access Violation errors but because of some other unusual errors that were flagged.