Events 2 and 3 on your screenshots are running on every tick. So they will be increasing WellenCounter and Welle 60 times per second or even faster! You can launch the game in Debug Mode (Shift+F4) and check these variables for yourself to see what's happening.
Create NewWave function and move the code to it.
If you need to start a new wave, say, every 20 seconds - use Timer behavior and call the function in "On timer" event.
If you need to start a new wave when no enemies left, then do this in an event where an enemy is destroyed: check if it was the last one and call the function.