Thank you everybody!
I solved it. My misstake was indeed the wiring. I went through everything and and it wasn't the A4 and A5 which caused the problem but Pin 0 and 1 and also just because I wanted to see the analog values on my screen. After just using the A7 value internally only and not starting any serial communication pins 0 and 1 were working properly again and my simple programm switch potentiometer is now working as well. Thank you very much.