OK, that is more useful:
Sep 16, 2022 17:37:11.205 [0x7f5d5e713b00] ERROR - [Notify] Failed to add watch for "/volume1/Videos - Dogs" (13: Permission denied)
See Synology FAQ - Questions, Answers, and a few How-To's - #25 by ChuckPa
and FAQ 19: Setting permissions to allow Plex access to media files
