I have an update (Sorry for possible double-post):
I’ve stumbled across this post: https://diydrones.com/forum/topics/pixhawk-i2c-not-working
I thought I’d give it a try and to my surprise ALL controllers are working!
So the good news: I didn’t break anything.
And the bad: Why does the compass stop working once I plug in the battery and how can I avoid that, so that I can fly… Secondly: What do I do with two spare Pixhawks? but I’m pretty sure I’ll find a solution to that problem 