Times go fast...
So, I did not dismantel the dryer but I removed the litle plastic cover under the condensor, checked the float, and it moved freely. Filled the sump with water, let it get pumped up,still same fault.
So, wonders of the internet, we already ordered a replacement.
A Bauknecht heatpump dryer as it was well priced and efficent and my mum trust in Bauknecht. Again purchased at OTTO and by some messing with cupons we got it for 589€ including delivery and setup. It is out of the new UltimateCare line, 7kg, A++ and with transparent door. Got the cycle setup with the seperate eCotton label cycle and the Reverse+ option as well as the XL- and WoolPro-cycles.
Lets see what this will get us into.