So I started with a fresh 5.5 installation of OMV using existing EXT4 disks, which were in fact previously also used with an earlier version of OMV (4.x).
However, now it occurs that whenever I mount a disk using the UI it is mounted in /srv/dev-disk-by-uuid-<someUUID> instead of /srv/dev-disk-by-label-<label>
Without success I have tried changing the label multiple times using e2label. Manually changing FSTAB of course did what I expected to achieve, but this breaks any further usage of the mounted devices in the OMV WebUI.
The problem seems similar to this one Change mountpoint from dev-disk-by-id to dev-disk-by-label which apparently ended without any satisfying solution.