Access complains : Update or CancelUpdate without AddNew or Edit

Hi,

I've build an application with Access 2000/2003 that uses a remote PG
8.2.6 via SSH tunnel.
The forms have bound controls and usually work decently.

But now and then a user reports a error (I have to translate):
... event before update caused an error:
Update or CancelUpdate without AddNew or Edit

They say they dont do anything special. Just clicking a checkbox or
trying to jump to the next record would do to trigger the error.
Later the same action won't cause any trouble, so I can't crack this in
debug-mode. Here on my box it always works as expected.

The error appears daily so the productive folks get annoyed but I have
no idea where to look.   :(

Could someone here give a hint?