Thanks, from your suggestion I made the changes to /etc/default/nfs-common and started idmapd, and the problem was resolved.
@Tiemen:
Thanks for the link, but even the wiki authors seem confused about this issue...
So, now it seems clear that it is a default configuration problem rather than a bug. I suggest this bug report should stand as a request to change the default configuration. As it currently stands it breaks NFS setups that were working in previous versions of Ubuntu.
@Perry:
Thanks, from your suggestion I made the changes to /etc/default/ nfs-common and started idmapd, and the problem was resolved.
@Tiemen:
Thanks for the link, but even the wiki authors seem confused about this issue...
So, now it seems clear that it is a default configuration problem rather than a bug. I suggest this bug report should stand as a request to change the default configuration. As it currently stands it breaks NFS setups that were working in previous versions of Ubuntu.