Jul 25, 2021 12:02:49.121 [0x7000018aa000] DEBUG - Completed: [71.205.168.204:48747] 403 GET /library/parts/210856/1608270166/file.flac?download=1 (15 live) TLS 3ms 288 bytes (pipelined: 1)
Is this a shared user? It appears you need to now enable the “downloads” permission for them in order to use Plexamp.
