I have had a mergerfs pool since OMV5 (now OMV6) all working fine (rpi4 with 4 USB drives).
Since upgrade to OMV6 though, there is some strange behaviour:
After every reboot, the mergerfs pool is not available in File Systems.
I then have to go into Mergerfs Plugin and manually click "Restart pool", which gives a browser error (see below).
After that mergerfs is available in File Systems and I need to restart Docker and all works well.