Error when advancing records in form

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Using Access 2007, I created a database with a data entry form and reports.
It works well. However, when an Access 2003 user opens the data entry form
and tries to advance the records (from navigator at bottom), they receive
this message:

"The value you entered isn't valid for this field."

Nothing is entered, they are merely advancing the records and it doesn't
matter which field the cursor is in.

2003 users can access the reports just fine and I've check the "Trusted
Locations" and those settings are correct.

Any ideas?

 

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We discovered more to the problem. The user gets this message whenever using
the navigation buttons for records - i.e., going to next page on a report as
well as going through the records in the data entry form.