OK, we swapped bottom modules, and it didn’t change anything! 
His bottom-module-PCB was bent similar to mine, so this obviously doesn’t matter.
I checked it before, and I thought this gap should be ok…
…but I gave it a shot with three layers of 80 g/m²-paper:
And that seemed to be it! 
Reboots are gone so far, and some drop tests didn’t lead to reboots. 
Thanks for the tips here! 