Hi guys,
thank you for your support. I finished the printjob. What I can say is, that I have more problems with warping near the corners than in the middle. Maybe my build Plate is not 100% leveld like Nicolinux says. But the build plate isn't bent.
The salt water methode works better than glue or hair spray. But not as good as printing with pva under the pla parts. But I'm with geert_2 that it is the best method to clean the build plate only with warm water.
When I have to print these parts again I want to try a material that has not this much shrinkage. Maybe PETG or pla pro/+. Or try to do less infill.