So, Day 1 with a fresh Debian Testing install. It has completely locked up my PC three times thus far: at this point, I'm thinking there might be something wrong with my PC! No logs to help me out, of course: just sudden and unpredictable system freezes requiring the use of the power switch to resolve.
Also, this is a bit of a show-stopper:
Clementine has long been my music player of choice and it has always been able to 'scrobble' details of the music I play to the last.fm website. Only now, it can't! The error message you see there implies that last.fm is sending a partially plain http response when a secure https message is expected, which would make it last.fm's fault (I guess). On the other hand, I did find a report which suggested that Debian itself might have removed things that break this particular feature.
In any case, happy though I am with the ease of setup of this particular Debian installation, no scrobbling is a deal breaker… and I've prepared a Fedora KDE live USB to test things out on instead. If it scrobbles, it's replacing Debian.
Updated to add: the live USB of Fedora doesn't include Clementine, but it's easily installable… and once it's running, it can't scrobble either. It's clearly a problem with this version of Clementine, rather than a Debian or Fedora issue, therefore. Debian can thus stay (at least until the next unexpected system lock-up)! Meanwhile, I need to find an alternative, scrobbling music player for KDE… (I shan't hold my breath).
Further updated to add: The problem isn't Clementine, but Firefox's built-in security mitigations. A description of how to fix Clementine's inability to be authorised with Last.fm is to be found here.