This problem seems to have been
solved. I believe that the issue was a misconfiguration of the
NFS server. I had:
/home/rikiheck/files
192.168.1.0/24(rw,sync,no_subtree_check,fsid=0)
/home/nancy/files
192.168.1.0/24(rw,sync,no_subtree_check)
/home/photos
192.168.1.0/24(rw,sync,no_subtree_check)
/multi
192.168.1.0/24(rw,sync,no_subtree_check)
/git
192.168.1.0/24(rw,sync,no_subtree_check)
But /home/rikiheck/files was an
ordinary directory that I want to export, not the root for
NFSv4. It was being mounted as NFSv3 (trying to mount with nfs4
would fail). But I'm guessing that it was being treated
inconsistently between the client and the server. Removing the
'fsid=0' tag solved the problem. It's still a bit of a mystery
what exactly the firewall was blocking, but it wasn't a firewall
problem, in the end.
Thanks to everyone who offered
suggestions and advice, especially Ed Greshko.
Probably I will take the opportunity
to move anyway to NFSv4, now that I've done all this reading
about how to do that!