This is Debian bug #400734: man 8 mount.cifs suggests that a credentials file can only contain username and password fields. It can, however, actually also include a workgroup field. Which is important if a Windows file server suddenly stops accepting user@domain as username but insists on username user and workgroup domain.
Created attachment 2406 [details] Very crude patch to fix this bug Attached is a (very crude) patch aimed at fixing this documentation ossue.
*** Bug 4713 has been marked as a duplicate of this bug. ***
Fixed with commit 1f7e09ea in master. Will be in the next releases. Thanks for reporting!