At some point, my Ubuntu desktop started using NFSv4 to connect to my Solaris file server.
The visible symptom caused by this switch was all files showing up as owned by 4294967294:4294967294.
The fix turned out to be to edit the file /etc/default/nfs-common to change:
Also, change the Domain= line in /etc/idmapd.conf to match your networks domain line.
Then reset idmapd
sudo restart idmapd