This isn't problem with foreign key constraint.

Processing error encountered while checking a record's primary key items for uniqueness.
The table associated with the current block of the form does not exist, or you do not have authority to access the table.

1. Check priviliges for all tables in application for user A and B again.
2. (This is my suggest only) in foms applications use full objects names (schema.oblect). and u will have much less headace with grants and priviliges.