It looks like the casting changes in Plexamp 3.4.6 on Android broke casting for me, for groups and individual speakers alike. I’m on Android 12 B1 on a Pixel 5, and I’m pretty sure the beta OS didn’t break this - but I can’t swear on that timing I have various Nest and non-Google devices, neither of them works with Plexamp, but they all work with Spotify or YTMusic.
My logs are like this, a bit redacted
May 24, 2021 08:47:03.302 [Javascript] WARNING - DEVICE: Player connection https://...d153fb3c8.plex.direct:10157 didn't work for Chrome: HTTP status -28
May 24, 2021 08:47:03.302 [Javascript] WARNING - DEVICE: Connection testing failed for Chrome
May 24, 2021 08:47:03.495 [Javascript] INFO - Cast: Session started.
May 24, 2021 08:47:03.495 [Javascript] INFO - Cast: Session started.
May 24, 2021 08:47:03.496 [Javascript] INFO - Cast: Starting channel.
May 24, 2021 08:47:03.536 [Javascript] INFO - Cast: Successfully connected to cc80b89f...
May 24, 2021 08:47:03.536 [Javascript] INFO - Audio Route: Switched over to player Both Speakers (volume: NaN).
May 24, 2021 08:47:03.536 [Javascript] WARNING - DEVICE: No provider for source e77fe7527b2...
May 24, 2021 08:47:03.537 [Javascript] WARNING - DEVICE: No provider for source e77fe7527b2...
May 24, 2021 08:47:03.537 [Javascript] WARNING - Cast: Could not find server e77fe7527b2... (exists: false).
May 24, 2021 08:47:03.537 [Javascript] INFO - Audio Route: Play failed.
May 24, 2021 08:47:03.537 [Javascript] INFO - Cast: Sending message 'STOP'.
May 24, 2021 08:47:03.537 [Javascript] WARNING - Audio Route: Error switching player to Both Speakers.
May 24, 2021 08:47:03.544 [Javascript] INFO - TypeError: Cannot read property 'clientIdentifier' of null
May 24, 2021 08:47:03.648 [Javascript] INFO - Cast: unknown player state.
May 24, 2021 08:47:03.750 [Javascript] INFO - Cast: unknown player state.
May 24, 2021 08:47:04.009 [Javascript] INFO - Audio: Not processing state update of stopped for track .