I think it could be the same problem I had with the encrypted drive -
for OMV4 drives need to be mounted to /srv/dev-disk-by-label-XYZ (XYZ obviously the label of your drive)..
My btrfs pool is mounted and accesed via /media/XXXXXX/. This is the path used by all the system (shared folders, plugins, tc.), but it is also mounted at /srv/dev-disk-by-label-Almacen, as you can see in the df output... (line 11). I can access it via ssh and this path.
overlay 60675128 32317812 25252088 57% /var/lib/docker/overlay2/d14e1347b2e40764d8de090849c651dc5cbaaa6cf3da70776897cf70f3974106/mergedshm 65536 0 65536 0% /var/lib/docker/containers/d469ebbf6cea6f9e2c58a43f11c88ba93d4a59f7e0244a47412e73f031d79827/mounts/shmoverlay 60675128 32317812 25252088 57% /var/lib/docker/overlay2/7b6fa438111440cf10fa0e94402bd38e9200db8ccda031a0d2659cdc3746d1dc/mergedshm 65536 4 65532 1% /var/lib/docker/containers/be9f23e924d9eca8e44bc6ac33941ab6a36f114c30467f67c5ad55332329d937/mounts/shm
It seem like OMV is not being aware that the pool is already mounted. (The error thrown when trying to mount form webui points in that way) (see image in first post).
On the other hand... lsblk -f output shows the pool mounting point at sdc (the pool has three hdds: sdb, sdc & sdd), but the error gived by the OMV webui mount command shows that cannot mount sdb...
May be the problem related with this?