Hi,
I was able to solved the problem myself.
Never use // in the /etc/fstab with NFS4. If you do so then the /etc/mtab
and /proc/mounts start to differ.
With NFS3 it seems to work correctly.
My workaround was
cat /etc/fstab | sed s/'\/\/'/'\/'/g > /etc/fstab_changed ; cp /etc/
fstab_changed /etc/fstab
cat /etc/mtab | sed s/'\/\/'/'\/'/g > /etc/mtab_changed; cp /etc/
mtab_changed /etc/mtab
best wishes
David