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