Unknown character &1.

What causes this issue?

Character &V1& is unknown.

System Response

Warning. The system displays the character '#' instead.

How to fix this error?

Do not use this character in the given environment (character set,
codepage).

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.