1st: try to install a newer version of the Plex Media Server. Version 1.18.5 is… pretty old (it was release in January 2020) – so there could be a number of things that might already have been fixed in the current public releases. If Synology’s package center isn’t offering you a newer version, please download the latest public/stable build from Plex’ download page:
Media Server Downloads | Plex Media Server for Windows, Mac, Linux, FreeBSD and More → make sure you pick the Intel 64-bit package for your NAS!
If that doesn’t fix things for you, please provide your server log files (alternative link on how to obtain the files manually if PMS isn’t running on your Syno)
