I fixed the problem by rewrite my hosts.allow and exports. To be honest i can't remember what the looked like a month ago. But i checked that the entire submask with every ip-address was written. Clients and fileserver is in different subnets.
It's still strange that it worked for 3-4 days and then i had to reexport my exports. But i guess that's the wonders of computers.. ;-)
portmap: 172.16.0.0/255.255.255.224 172.16.0.32/255.255.255.248 127.0.0.1
/Apps 172.16.0.0/255.255.255.224(rw,sync,no_root_squash) 127.0.0.1(rw,sync,no_root_squash) 172.16.0.32/255.255.255.248(rw,sync,no_root_squash)