Plain lvm user here, had the vg for about a year without changes and without issues. Root mountpoint has always been 2775.
Tried OMV last night, seemed to be working fine. Looked today and found that only SOME of the users group could write and some could read. Checked and doubled checked everything, set 777 on everything on the lvm, set owner to root:users (as suggested several times in other threads) etc etc etc.
After many hours root (or any other standard linux user) can ls the lvm, see all contents including permissions, but nothing installed by openmediavault can.
Good luck to everyone else!