Dear Johan,
Please check the following steps:
- Go to transaction SU01
- Open the user which is being maintained in the "User Name" field
- Go to tab "Logon Data"
- Please check if the Alias for this user is maintained.
When the Alias for a user is maintained, the field in transaction BP will always remain grey out and you cannot edit it.
If you wish to delete or change the field value, then you must remove Alias value for this user from transaction SU01.
Hope my answer helps.
Best regards,
Julin Xin