Thank you very much for all your Feedback!

I have been printing different models now and simply increasing the minimum layer-time seems to work. So the problem realy was my impatience :-)

Solutions: increase min-layer time, print purge walls, print a cylinder.. or (my favorite) just printing multiple objects (of nealry equal height).
I will try to experiment with those.

@temps / slurry
Since it was suggested, i thinned the slurry a bit and lowered the temperature.
I am down to about 100°C. I will go on lowering the temp with the next prints until i run into problems.

@ fan-topic:
I think this would depend on the fan. If your fan is too powerfull, it might cool the plastic, before it has a chance to bond with the layer below. So under certain circumstances / with certain printers, this might really be a thing, i guess.
So both of you seem to be right.

All in all, my problems seem to be resolved by now. :-)
Thanks