The other machine (an NFS server) is running 4.1.x and needs a patch from Sun to update its network lock daemon (lockd). If you don't install the patch on the server, file locking will not work on files mounted from "thathost". The lockd jumbo patch fixes a bunch of other lock manager problems, so it may be a Good Thing To Get; however, it may also cause the machine on which the patch is installed to have trouble talking to servers with no patch or older patches, so Be Warned.
The lockd patches are: 100988 (4.1.3), 101817 (4.1-4.1.2) 101784 (4.1.3_U1), 102264 (4.1.4) and 100518 (for Online: Disksuite).
Make sure you install the latest version of those patches.