After switching from Debian Buster (installed via Petitboot) to Armbian, I have problems with NFS on Openmediavault that didn't exist the weeks before. My NFS streams abort after a few minutes, sometimes seconds (on Libreelec as well as Kodi for Windows). In OMV it shows me that the service is enabled and running, but systemctl says: nfs-server.service loaded active exited. With exportfs -rav it runs again, only to stop after a few minutes. It also works without command if I wait a few minutes. I also did a fresh install of Armbian, wich did nothing.

today I reinstalled with kernel 5.9.14 - and all seems well again. It also fixed other problems, with 5.10 copy operations with Samba or Rsync were continuously slower (sometimes down to < 1MB/s, instead of 50-110), the CPU load was higher, the SD card with the OS often went empty during copy operations...

It could be BTRFS related problem. Since this is kind of mainstream FS now, it could be fixed pretty quick. But we can't tell you when ...