I've been fighting this for the past week or two as well-- SMB shares are unaffected, but streaming over my NFS (v3) shares causes disconnects for audio and video files.
The only errors I've seen are "EOF" (end-of-file) errors which suggests that the client asks for the next bit of the file, and server says "Sorry, that's it", and I haven't found any errors in the logs on my OMV system, so I'm not seeing the same error message parading lunatic is, but I'm also running NFS v3 (over TCP to one box, and UDP to another). Both the NFS clients haven't changed software in weeks, if not months.
Since this started when I did a full update a week or two ago, I went back and looked at the logs for what had updated, and the only package that would be likely to affect NFS service would be the linux kernel.
I booted from the older kernel (4.19.0-0.bpo.1-amd64) instead of the current kernel (4.19.0-0.bpo.2-amd64), and I've been unable to duplicate the problem since.
I'll update if it fails again, but so far, it's looking pretty solid.