[AccessD] Field Cannot be Updated

I am creating a form for vendor information, it contains a tabbed form with
the regular vendor data from tblVendor. The 2nd tab comes from tblType -
tblType lists the different types of businesses the vendor can be (Small,
women-owned, etc). The last tab is a subform for adding the vendor contacts
(tblContact).

When I make a new entry and type something in any of the fields on the first
tab for Vendor information, I receive - Field Cannot be Updated - I have
never seen this message before. After several clicks of OK, the data I typed
in the field shows & the record works just fine - all the other fields can
be updated and the record shows all the data.

What & where should I look to find what is causing the error?

tblVendor has VendorID PK
tblType has VendorID FK
tblContact has VendorID FK

I have Cascade Update & Delete checked - would this have something to do
with the error? I have used it before without errors.

Virginia