nfs: can't get a request slot

[Debian Mail <debian@sos.ethz.ch>]

I have two machines on different networks. When I mount a nfs volume,
it seems to work and I can cd to it, but when I want to copy a file
from it I get:

nfs: server nfs.server not responding, still trying
nfs: server nfs.server not responding, still trying
nfs: task 416 can't get a request slot

And the copy process hangs.
The nfs server is comming from nfs-server_2.2beta37-4.deb
(Before I had nfs-server_2.2beta37-1.deb with the same error)

Any idea what could be wrong?

Stef