David,
Can't give precise advice, but it looks like you have macro or code
"somewhere" which is trying to change the value of a field or control.
You will just need to scout around and see if you can spot it. Look
at the BeforeUpdate and OnCurrent properties of the form, and the
properties of the last control on the form that has the focus before
you try to go to another one, or the properties of the command button,
if you use one to go to a new record. If it's not there... it must be
somewhere else
- Steve Schapel, Microsoft Access MVP