When removing a Samba server user with "root" system user privileges via Plesk, an error occurs:
Fehler: FileServerUser::delete() failed: FileServerManager::del_user()
failed: Unable to del user: filesrvmng failed: filesrvmng:
/usr/bin/smbpasswd execution failed:
Failed to delete entry for user root.
filesrvmng: Error during execute '/usr/bin/smbpasswd -x root'
This is Plesk internal issue with id #PPPM-3856 , which is planned to be fixed in future Plesk updates
Such an error can be safely ignored.