From Mexico.purplecow.org
Solaris 10, Linux modernish, NFSv4 mounts fail
At some point, Linux stopped cleanly mounting NFSv4 shares from a Solaris 10 host cleanly. The error message in syslog is
nfs4_discover_server_trunking unhandled error -5. Exiting with error EIO
with "mount system call failed" as the terminal output.
The fix is the add the following mount options:
vers=4,minorversion=0
And the mount should be clean.