iPad 3 4G dead after battery change

The iPad still worked fine, but the battery was bloated so I changed it following this instruction: https://de.ifixit.com/Anleitung/iPad+3+4...

I thought it went well. No broken glass, no battery exploding in my face and no obviously broken cables, but after reassambling, the iPad is dead. No charging symbol, not even trying to boot, no sound, nothing.

I tried to start directly, to load it from a computer and from a socket.

So, before I accept my defeat, does anybody have an idea what I could have done wrong? Something worth checking, even if I just learn that I broke one part beyond repair.

Thanks for your support.