The second screenshot, where you have the permissions listed, is wrong. You should NOT have a “plex” user. In the “local users” dropdown, select “System internal user” and apply permissions to the PlexMediaServer account. That is the correct and only way to do it.
Remove the “plex” user. Completely. Forget it ever existed.
Try that as the first step to see if it resolves the issue, or at least gets you further.
If you join in the conversation on this thread Synology - DSM 7 Release - Plex Installation Issues - #1301 by ChuckPa you’re more likely to find assistance. That thread references this link (DSM 7 - Plex Media Server - Release Info & Migration instructions) which is the complete, step-by-step detailed instructions from the man who put the package together.
