Strange Debian NFS server problem

This is a strange one I can't find in any NFS troubleshooting guide or tutorial. I really need help for this, since I have a lot of personal files on my server I need to get access to. I did have this working at one point, except I think I installed or uninstalled something else that had the side effect of messing up NFS

My debian/sparc NFS server seems to be installed correctly, and when I type:
showmount -e on the client after restarting nfs-kernel-server on the server, the NFS mounts are shown correctly.

rpcinfo -p shows v2 portmapper, v1 status, v2-4 nfs, v1,3,4 nlockmgr, and v1-3 mountd
But when I try to mount any of the exported volumes, I get nothing but timeouts and retries on the client which eventually gives up, (15 seconds timeout, which should be more than adequate)

After the failed mount attempt, showmount -e no longer shows any mounts.

Question: How do I check that a necessary port is not blocked? If rpcinfo and showmount pass, doesn't this mean that the ports are open?

I have taken everything out of hosts.allow and hosts.deny but it does not solve the problem. I think this is some sort of security or firewall related problem, but I am out of ideas.