Plex cannot start, permission errors

[OttoKerner]

Not a Synology expert here, but this sounds a lot like

Plex Media Server security changes for Synology users

Community Announcement - Synology users - Security update. Server Version#: Plex Media Server 1.15.4.994 and above By popular customer request and to improve the safety of your media, we’ve made a small but visible change to what PMS can do on your NAS by itself. We have removed user plex from the administrators group. This was a holdover from DSM 5.2 when it was required for PMS to run properly on the NAS. Now, with DSM 6 and above, this is no longer required. By doing so we’ve lower…