Error message about long interger field size and ValidationRule

I have two copies of a database (entry 1 and entry 2), the second an exact replica of the first, but with different forms entered into them (ultimately both will have a complete set of forms, but the two data entry staff are working from opposite ends of the raw data).

In two particular fields in entry 2, the database will not allow us to enter any three digits above 200 (i.e. 0-200 enter fine, 201+ I get the error message). There is no problem in entry 1 in entering digits above 200 in the same fields. The Field Properties are the same for both - long integer. There are no validation rules for these fields.

I am getting two error messages - The first says:

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

For example, you may have entered text in a numeric field or a number that is larger than the FieldSize settin permits."

When I click "ok" and try to close the form, I then get:

"The expression [CHIFFRE] you entered in the form control's ValidationRule property contains the error The object doesn't contain the Automation object 'CHIFFRE'.

Microsoft Office Access can't parse the ValidationRule expression you entered. "

I have looked online and spoken to colleagues to no avail.

Any ideas gratefully received.

Thanks