I just happened to experience this same issue after migrating to OMV5. I have 3 drives pooled, placed Plex's /config and /transcode in a share in the pool and it did not worked. Now, I know that I have to put those folders outside the pool, but how? I am considering two options:
1. Put both folders in one drive of the pool and point Docker directly to them (use the drive_ID/config instead of pool-mount-point/config). However, I am not sure if that might mess up with the pool. I tried that already and it worked, but I ended up with two folders with identical drive names under /srv: one with the data from the pool and another one with only the Plex Database. Any ideas or recommendations?
2. Put both folders in the OS drive under /var/lib/Plex. I am using a 64 Gb SSD and the database should not be that big (circa 3,5 GB)
3. Any other suggestions?
Thanks in advance