Seaquel
SQL Server error code

SQL Server error 6903

SQL Server raises error 6903 at severity 16, which marks a problem with the query or data that you can correct.

XML Validation: Invalid definition for type '%ls'. SQL Server does not currently support inclusion of ID, QName, or list of QName among the member types of a union type.
Msg6903
Severity16
Group6000–6999

Source: Database Engine events and errors