This error popped up out of the blue about a week ago and I found this thread (it didn't help me, but hopefully this followup will help someone). I've been using FreeBSD since its beginnings and have never seen this error before. Both statd and lockd were enabled on the server and all clients, and they had all been working fine for some time. I eventually traced the problem to a badly corrupted installation on one of the client machines (by shutting everything down and fully testing NFS after bringing each one up). The problem client was causing stale locks, and hence problems with writes from other clients.
The hardware reliability of the corrupted client is uncertain (it's newly acquired and several years old). We were also hit by multiple power outages in rapid succession recently. After bringing this client back up, it exhibited other serious issues. After 43 passes of memtest86, a BIOS flash, and a clean reinstall, everything is back to normal (for the moment, but I won't be surprised if this client machine deteriorates again).
An important lesson I've learned over the years is that if there's any inconsistency in a FreeBSD system's behavior, it's best to look for a hardware issue. FreeBSD isn't perfect, but it sure is predictable. I've never been able to conclusively trace flaky behavior back to the OS, but I have fixed a lot of flaky problems with hardware replacements, and occasionally with BIOS updates.