You’ve exceeded the number of directories Linux can monitor (the error message says “no space left” but in reality the inotify table size has been exceeded).
Aug 01, 2022 21:45:13.665 [0x7ff3223d0b38] ERROR - [Notify] Failed to add watch for "/volume2/Music/Cdot Honcho/Cdot Honcho - OTW" (28: No space left on device)
See FAQ 16 for the procedure to increase the table size.
FAQ 16: Increase the number of folders Plex can monitor
Note that this is a Linux, not Plex, setting. You may have to change it again when Synology issues DSM updates.
