From blog post on 17. May 2019 by volker :
A new ISO image based on OMV 4.1.22 has been released. The ISO image now supports UEFI.
However I tested this version (burned with Etcher), when setting my system to UEFI mode it boots but screen freeze straight after boot with lines on screen "video mode not supported".
Works fine in BIOS mode.
Dell Poweredge r410
OMV 4.1.22
edit : works now after burning ISO with rufus (same USB key). albeit network cannot be configured now but I don't think it is related.