Solution: So it turns out FUSE is actually the culprit....and the answer was right here in the LinuxIO Plex support thread. Apparently FUSE freaks out when the cache status for a share is set to cache only and the appdata path for the container is set to /mnt/user/appdata instead of /mnt/cache/appdata, it would be nice if this was included in the FAQ topic of somewhere easily findable instead of having to google "docker shm error" and hunting through tons of random posts.      I h