I checked the valvebody gaskets I had in the trans again and found that no hole in the seperator plate was covered, so that shouldn't be the fault.
I called the tech support of Transgo and the guy was very helpful.
I told him the results of the pressure test and he found them kind of strange.
He also said that the pressure valve has no effect on the problem that I have.
He wants me to do additional pressure tests at 1500 rpm in OD and D to see if the pressure rises.
I also called a GM dealer about a new pressure valve. They want over 60 bucks for it. I saw it online for 9$ somewhere. Crazy germans!
If anyone has ideas please share them.