Problem solved, I just don't know how
I tried changing a few settings, but it still didn't work. I tried from the live image (everything worked fine), I tried with a new user (everything worked fine) and then tried as myself again (and everything now works fine). I have no idea if it's because I changed anything, or just because I rebooted (although it had already been rebooted since the Petra upgrade).
In any case, thanks for your suggestions!