I noticed Media player also showed zero songs for every playlist, but still played the songs while showing their length as 00:00. And ä/ö/å letters in file names were messed up.

I copied one video from SD card to PC via USB cable and copied it back to SD card. Seeking the video started to work. I copied another video and it worked too. But soon problems came back for the first video, and then the second video, in a matter of minutes. At the moment I can seek the first video...

Diagnosis: SD card file system has got the flu.

Planned treatment: Copy all files back to PC, format SD card, copy files back (or buy a new SD card). Remember to always remove SD card when reseting or updating the phone, so the SD card won't get infected.

I also installed Patchmanager and tried the Gallery video player tweak, seems to work well.

Indeed I've been using the original Jolla charger. My Nokia AC-10's connector is broken. Time to upgrade to a Samsung 2 Amp charger