No user exists with SNC name "&"

What causes this issue?

A dialog logon occurred using Secure Network Communication (SNC). The
system could not find an R/3 user in the table USRACL for the user name
passed from the external security system to the R/3 System.

System Response

Logon is refused.

How to fix this error?

Use user maintenance (SU01) to assign the SNC to a user as an external
name.

Error message extract from SAP system. Copyright SAP SE.