Okay, this is VERY weird.
I unplugged all the Cables, unplugged the module and plugged it in a different power socket in the wall (just because I wanted to work on it at a different place in the room). Now its booting and everything is fine. Not sure if or why that makes sense or why it works. Thought about deleting my whole question but maybe this helps somebody in the future.
"Have you tried turning it off and on again?"
I unplugged all the Cables, unplugged the module and plugged it in a different power socket in the wall (just because I wanted to work on it at a different place in the room). Now its booting and everything is fine. Not sure if or why that makes sense or why it works. Thought about deleting my whole question but maybe this helps somebody in the future.
"Have you tried turning it off and on again?"