As promised (and much delayed, because I wanted to reproduce one more time, but since I don't use that alarm playlist anymore, I won't be), my theory as to why streams fail to play in some cases:

See https://bugs-archive.lyrion.org/show_bug.cgi?id=9534 and especially the attached log files from it. What seems to happen (at least as of SC 7.2, maybe later) is that when it tries to play a playlist, it hits every URL in the playlist, gets its content type and other info, and adds it to the db. Then it picks one to play.

If the initial scan of a stream returns a 200, but the actual play attempt returns a redirect, SC will try to redirect to the Location: URL, as it should. But the code expects the URL to be in the db, so it fails to instruct the squeezebox to stream and ends up in never never land.

As reported in the other bug, the alarm code doesn't detect the lack of playback, and thus the alarm doesn't resort to any of the backup alarm strategies. But, this bug is about the failure to play a stream in this case.

I'm not actually sure that this bug is still present in newer versions (because I think it only gets triggered at times when I use it as an alarm, and I need a more reliable alarm, so I don't test this anymore). I've seen no indication that this bug was fixed though.