Some times when I have tried to play movies from my Plex Server, the server monitor is filling with an error message.
Now when I tried to play a movie from my Plex server this line was raging through on my server monitor.
Managed to take a picture while it was filling the screen with nothing but this line over and over.
94 printk messages dropped ** [70053.896705] ->rcu_read_unlock_special: 0x1 (b: 1, nq: 0)
Disable the plex server and reboot the system and check if you still see the messages, what about if you just boot up to run level three (systemctl set-default multi-user.target)? That should isolate things a bit to see if they are still present, login as root user an follow the journal (journalctl -f) for a bit.