Thanks man! Great tutorial! Your solution deactivates the trigger and recreates the victory conditions etc. manually. I might need that in case I run into problems with the Game Map initialization Trigger again.
I also posted the question in the SC2mapster Discord channel and the user 'Talv' had an easier solution for my specific problem: There are triggers to turn the XP Gain back on. This was all I needed and I can leave the default Game Map initialization Trigger active.
MeleeSetOption(lv_player, c_meleeOptionXPGainDisable, false);
Still thanks again MaskedImposter.
This thread can be marked as SOLVED.