Problem solved!
First, I forgot to activate the internal Pullup:
digitalWrite(DIGITAL_INPUT_SENSOR, HIGH);
Besides that, every signal greater than 0.5V is recognized as interrupt, BUT digitalRead only recognizes signals greater than 1.5V!
When my external system triggers, it starts to slowly increase the voltage from 0.5V up to 3.3V. It reaches 1.5V after 5 seconds triggering. I cancelled the test every time after 3-4 seconds, so that's the problem why nothing worked.