Hello once more!
Although the problem is solved, i wanted to share the end of the story. Maybe it is some kind of helpful for others as well.
After identifying the DP cable as the source of all trouble i contacted the ASUS support for a replacement of the defective one. The Asus employee agreed in the perception, that such problems, caused by cheap quality accessories to an otherwise premium product, are bad for the customer as well as the company. However, he also made clear that ASUS itself grants no sort of (implied) warranty for the supplied cables, which by law is ok (talking about EU/Germany).
In my country we have some other kind of (express) warranty from the retailer (not the manufacturor) that has to be granted for two years in all cases. So i contacted Amazon in that matter, described my experience with the ASUS support, and they offered me three possibilities:
- Send back everything and get a refund
- Send back everything and get a new monitor
- Refunding of 10% of the initial cost (~65€). So some sort of retroactive discount.
Because i'm very happy with the monitor itself, and didnt want to go through some kind of lotterey with display quality, i agreed to the latter.
tl;dr: Asus support couldn't help and can be considered subpar. Amazon support is delightful.