I'm running OMV6 on my RP 4b NAS consisting two 2Tb hard drives. I also have moode music streamer on a RPI 3b.
In OMV6 I have two shared folders, Backup and Media.
Media is then subdivided to Flac Music, Movies, MP3 Music, Music Videos, Photos, syncthing, Tvseries and Videos.
I have Plex and syncthing running in Docker/Portainer.
Plex server works well on my TVs and tablets and is always available.
Moode picks up the Flac Music files via a NFS share faultlessly also.
However, MiniDLNA has a problem showing my syncthing files (all image and videos from two phones)
When I first set up the Media folder share in MiniDLNA with reset permissions and file type "All", everything shows up fine on my tv via minidlna. After a couple of days the syncthing folder shows up on the tv but with no recognizable file type. Shortly after that even the empty syncthing folder disappears. All the other folders in Media are visible and accessible.
The only way I have found to re-establish the syncthing files is to reboot OMV6, reset permissions of the Media folder and then change the file type to image or video. When that process has finished, I then have to go back in and change the file type back to "All". Everything is then fine for a couple of days until it loses syncthing again.
To reiterate, Plex Media server works fine and all the syncthing files are accessible via sftp://192.168.68.xxx.
It's not the end of the world as I can still view my phone pictures via Plex, but it's frustrating and intriguing why MiniDLNA won't play.
I've been googling this problem for a long time but can't find any solution. Ideas please 
