Good morning Olias,
the pictures for your answer here seem to have got lost. I found them in the thread next door?! 
I clean the build plate regularly, yes. I’m generally pretty meticulous about getting a good result every time. I’ve just washed it again though.
Until now the bad printing only ever happens at the edges. The layer itself actually looks quite good. So I’ve just done the suggested test print:
Here the layer has a lot of ripples. I noticed that at the end of the test print, the nozzle hit the (printed) insertion aids of the build plate. So I removed them and the hotend and inspected it. Apart from a small bead of plastic, the hotend / nozzle looks good. Nothing is broken, loose or bent. Removed anyway I cleaned it straight away of course.
Next test print:
The edges aren’t perfect, but at least much better.
Regarding the ripples in the first layer to me it reads like a leveling needs to be done(?)