Error when updating a record in table &

What causes this issue?

Since the name of the accessed form routine &V1&_USER_EXIT in program
SAPFGLRU is determined from the user exit name &V1&, it can only
consist of letters or numbers.

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?

Error message extract from SAP system. Copyright SAP SE.