How to fix: Bluetooth headphones immediately disconnecting

The other day, I ran into a strange problem with my Bluetooth headphones: they connect to my PC, but immediately disconnect. When I open Blueman and try to re-connect manually, the same thing happens, and Blueman displays a message saying “Connection failed: Protocol not available”. The headphones stay connected to my smartphone the whole time, though, so it’s obvious that the problem is in the PC itself.

Almost by chance, I found out the cause: PulseAudio’s user daemon not running. That’s easy enough to fix:

$ pulseaudio --exit-idle-time=-1

For some reason, the daemon exits automatically after 20 seconds of inactivity, so I have to pass it a -1 to disable that “feature”.