Hi Sascha Wolkenar,
Sorry about the trouble, this should have been mentioned by now in the text.
Please try first to increase the Cloth Tag> Expert> Subsampling value.
This was discussed here as well:
https://www.cineversity.com/forums/viewthread/2463/
I have checked my file as well, and it seems to be fine with the current version [R20]
Have a look to this file, where you can find five different ways to connect the curtain:
https://www.amazon.com/clouddrive/share/fHzE6TATILST1qxuuT4ZqseWB6EHnjxDsLwqIYdRsgA
Please never worry about language, I know only too well how that feels. (If you are uncertain that a part of your communication might be not understood, write in () in the way you feel comfortable after the text, I will try to translate it (e.g., via online translation). (I speak German fluently, BTW)
Again, sorry about the inconvenience.
ENJOY.