With my Iomega ix2-200 Storcenter running kirkwood-dt kernels 5.10.11 or 5.10.12 I have an issue where nfs connections to it show many read errors in the client, notably in Kodi. However, a direct mount on MacOs Catalina appears to work fine. I'm not seeing any issues with dmesg or journalctl, where the later shows proper authentication (rpc.mountd[461]: authenticated mount request). Reverting back to 5.10.10-1 resolves the issue.
Some web searching points me to https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2460556.html and https://lwn.net/Articles/840129/, where there is some talk about nfs and https://lwn.net/Articles/842943/ shows quite a few bugfixes related to it.