With DSM 7, the Plex username changed from Plex to PlexMediaServer. It is also a system internal user (new for DSM 7).
Give PlexMediaServer read only or read/write permissions to your media folders.
See FAQ #19 & FAQ #30: Synology FAQ - Questions, Answers, and a few How-To’s
With DSM 7.x, the user Plex is no longer needed by Plex Media Server and can be removed from your system if desired.
Jun 30, 2022 16:18:59.434 [0x7f514f9b8b38] ERROR - [Notify] Failed to add watch for "/volume1/TV Shows" (13: Permission denied)
Jun 30, 2022 16:18:59.435 [0x7f514f9b8b38] DEBUG - [Notify] Can't open root directory "/volume1/TV Shows" (boost::filesystem::directory_iterator::construct: Permission denied: "/volume1/TV Shows")
Jun 30, 2022 16:18:59.435 [0x7f514f9b8b38] ERROR - [Notify] Failed to add watch for "/volume1/video" (13: Permission denied)
Jun 30, 2022 16:18:59.435 [0x7f514f9b8b38] DEBUG - [Notify] Can't open root directory "/volume1/video" (boost::filesystem::directory_iterator::construct: Permission denied: "/volume1/video")
