Bug#788546: Sorry for late answer, got no notification on your posting

[Ingo <alf.debianfan@gmx.de>]

Das mit den Locales hat es nicht gebracht, da ist sowohl auf dem nas als
auch auf dem PC die gleiche:

PC:$ echo $LANG
de_DE.utf8

nas: $ echo $LANG
de_DE.UTF-8

But a workaround is documented in this Bug
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=790880
which has been merged here. This states:

Using "mount.nfs" instead allows  mounting by a user:

Executing mount.nfs as a user with full path does not respect the
mountpoint as configured in fstab:

~$ /sbin/mount.nfs leo:/Bilder
~$ mount.nfs: no mount point provided

Adding the mountpoint works for both mount and umount:

~$ /sbin/mount.nfs leo:/Bilder /home/ingo/leo.Bilder
~$ /sbin/umount.nfs /home/ingo/leo.Bilder

That's why I assigned it also to Package: mount.

Today I checkes in Stretch: still the same issue.