Ok, I solved it myself. Here’s how (as documentation for others):
1. I burned a fresh image on the 32GB SD-card
2. I used another raspi w/o AMP2 card to boot this image.
3. This raspi booted fine and expanded the image to use the full disk space.
4. I set up the WiFi connection using the AMP-less raspi as the other always stopped when setting up networking
5. I booted the image with the original raspi and so got it working again.
Unsure, as to why the Raspi would not boot with AMP2 attached, but as problem is solved I will not ask any questions now.