When you issue 'mount.cifs' like this:

mount.cifs -o
credentials=/etc/samba/fstab_credentials,rw,uid=81,gid=9005,forceuid,forcegid
//ha-nas-int.web-factory.de/www_apache /server/data/www/apache_smb

You get 'mount error(22): Invalid argument
Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)'.

In the manpage of mount.cifs you can find:
"""
OPTIONS
...
forcegid
instructs the client to ignore any gid provided by the server for
files and directories and to always assign the owner to be the value of the
gid= option. See the section on FILE AND DIRECTORY OWNERSHIP AND PERMISSIONS
below for more
information.
...
"""

When you issue mount.cifs without 'forcegid', it succeedes (even with
'forceuid' in place).

This seems like a upstream-bug, because Google shows a bunch of eaqual issues over the distros (ex.: https://bugzilla.redhat.com/show_bug.cgi?id=850578 ).

Therefor, I files this issue in upstream (https://bugzilla.samba.org/show_bug.cgi?id=9219), also. It is in here to be found by people searching for this issue.