Entry &1 has invalid characters

What causes this issue?

The entry has invalid characters.
This is checked by a Business Add-In (BAdI) which checks each character
of the entry. A function module is used in the implementation which does
not allow this value to be entered.
For example, the function module checks whether the entry only consists
of letters and ends with a number (CCCCN) so that the values such as
AcSD2, gT3, and so on are allowed, but not values like D7AwF.

System Response

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

How to fix this error?

If you want to check the entry, the coding of the BAdI must be changed.
Otherwise, the BAdI has to be deactivated.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.