Well, I think the reason, and I may be speaking for myself here, that people insist on using Plex is that some of us have the liftime pass. So to not use it defeats the purpose of having it. I've had Plex since before I had OMV so naturally I installed it. It's a good platform if they would just get that plugin to behave.
Someone may be able to shed some light on this but the last system update to OMV changed some things directory wise right? I didn't have the issue till I updated OMV. Not saying it's an OMV issue, just that the developers of the Plex plugin are a little behind. Usually fixes don't take this long.
Maybe someone can shed some light on this. I'm used to having to tweak things with Linux so it was more frustrating finding the solution than actually applying it. But to those who just want it to work, well, I'm not sure what to say there.