Week & not allowed (internal error)

What causes this issue?

Week &V1& is not allowed (invalid character, not in admissible range,
etc.). The field may even be empty.
The error occurred in function module &V2&. It may be due to one of the
following reasons:

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

internal program error
faulty call of function module &V2& with invalid or inconsistent data

System Response

How to fix this error?

Contact your system administrator.

Error message extract from SAP system. Copyright SAP SE.