Hello,
I been using OMV3 (fresh installation) for two months now, and I didn't pay attention about this Emby/Plex video playback remotely until last week.
I tried several attempts with the old Share folder, but it doesn't work.
So, I created a new Share folder with default permissions = 2755, root:users; sub-folders = Zero:users. NO ACL.
Un-installed Emby, re-installed Emby, but no dice.
If I play the media files from local network, no issue.
Emby/Plex use the same share folder.
All appropriate ports are forwarding in the router.
When I play the mp3 or mp4, the files are stuttering/buffering.
It sometimes play if I let it buffering more than 5 mins.
I ran out of idea.