I researched some more and found this video of exactly the same problem and some Amazon reviews which state it, too. I ordered an inverter and will replace it myself and update if it fixes the issue.
My Bosch WAS28443 washer doesn't spin-dry anymore. Well... most of the time anyway. I had a mechanic coming over (5min for 70€) who said it was the inverter and suggested a repair for 250€. I don't really believe him, because I think the inverter is an on/off thing, and not "sometimes" or randomly.
It doesn't follow any pattern I could recognize. Yesterday I had a 6kg load and it worked flawlessly, today it was only light sports clothing ~1kg and it didn't work. Also, if I set it to spin only after it didn't work, sometimes it spins without further problems, sometimes it doesn't, no matter which speed, or any other settings are set up.
I uploaded a video (https://vimeo.com/224351376) to show the problem. As I wrote, sometimes everything is in order, but most of the time it tries to spin, but can't/won't.
I think it sounds like a cog not having enough "grip", lacking a better term.
I'm thinking of opening the machine, but wanted to ask some maybe specialist if this is a good idea or if I just should get a mechanic.