Windows event log contains nothing about these events. And strangely nothing in the webapp log eighter, there seems to be dates missing, maybe the log was turned off at the time. I've reseted the log file, it was about 11 MB. I'll check it again next week to see if there is some information in it.
The deployed code are identical. Maybe data, but I've tried to login to the customers web and tried to do the same thing as they did, without getting the error. It seems it happens randomly.
I'll come back with any info from the log next week.
Yes, it happens when a form on the page is submitted, and RequestSave is called. However, I've tried to remove records before saving, and I have tried to change to an rowid (in the hidden input in the HTML) that doesn't exist, but that results in an other error, still 25, but the row (in code) point to Find_By_Recnum and not Request_Save.
UPDATE 2010-03-09:
No trace of the errors in the new log, I'll check again later this week