A known issue, with temporary work around, on all ApolloLake CPUs (which your Syno uses).
- SSH into the Syno (PuTTy or ssh command line from Mac / Linux)
- the following sequence
sudo -su root
cd "/var/packages/Plex Media Server/target/lib/dri"
ls
rm -f iHD_drv_video.so
ls
From the two listings, you can confirm iHD_drv_video.so has been removed.
Now restart PMS.