Hi! This probably is not helpful for a resolution now, but perhaps the information is good to share.
We also started to see this error when we upgraded FAS6*** series heads to DOT 8.1.1P1 from DOT 8.0.2P5 7-mode (upgraded in Oct. 2012). We were accessing NFS shares using multiple versions of Linux OS, and therefore, it didn't appear to be specific to certain Linux distributions. We have seen errors on OpenSuSE and Cent OS at least.
The error was somewhat random, but we were getting it, and affected our several of NFS4 volumes, especially in a high load with certain software combination like OpenNX. Downgrade to 8.0.2P5 after talking to NetApp support was not reasonable (major downtime a few days+).
We worked with NetApp tech support for quite long time including multiple network traces, and NetApp determined it was a DOT bug. I was initially told that it would be fixed in the upcoming release version 8.1.3.
Based on what you reported, the bug appears to exist on 8.1.2 (or it has not been fixed as of 8.1.2). Our current work around has been to run affected NFS4 volumes on a filer pair running older DOT 8.0.2P5 version which may or may not be reasonable to implement in your situation.