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
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!