Hey, with these steps you can play the campaign as germans, including the skirmishes:
1: launch the game with "-entry_race germans_campaign"
2: start new campaign (disable tutorial! important!)
3: save
4: exit game
5: start game without arguments
6: start new campaign
7: load previous saved campaign
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Now you can start skirmishes / missions without the scar error.
I was not able to start the Pomigliano Airfield mission, regardless of US, british or german company attacking, so you have to naval or artillery bombard it until you can take it over with an infantry unit without a skirmish. (the "play" button stayed disabled on that mission for me) - counter repair Naples with an engineer / pioneer unit if u need more time. All following mission worked fine for me.
This works, thanks for sharing.