Hi Tom!
Just wanted to let you know, that I figured out what the problem was.
PHP memory limit was to low and this resulted in the blank page after I installed YOAST.
Mean thing was that if I used the English backend it was just enough to work and if I switched to German it wasn’t. I realized it when I switched to debug mode in WP.
So if anyone has problems with a blank page without any error message it problably is the memory limit.