So, when I bought this flat 4+ years ago, there was already a condensation dryer in the bathroom. Of course I regularly emptied the lint trap below the drum. Then I saw this comment https://imgur.com/gallery/OP9NNBf/comment/2467346439 and ... err..
... had a closer look at the dryer.
Found the cooling unit that does indeed act as 2nd lint trap and was covered in sopping wet lint.
Let this be your #servicepost