How to get right voltage output with Bts7960b H-Bridge?
Thanks to the hints of @Peter Karlsen I figured it out:

I was used to the 8 bit resolution for PMW of the Arduino boards. But since my NodeMCU uses an ESP8266 Chip the PMW resolution is 10 bit [reference]. So the max value is not 255 but 1023.

Changing the values in my code solved the problem.