Won’t turn on after battery replacement

After battery replacement it won't turn on anymore. No reaction on the screen, no haptic feedback - no vibration, phone offline in "find my device"

What I tried:

• plug in to charger
• long press power on
• long press power on + volume down
• even tried to reconnect the old swollen battery which worked before, but with the old battery which worked an hour ago it's also dead.
• I fully reassembled the device in case it does some integrity checks before booting, (Fingerprint sensor, etc.)

Any ideas what I can try?

I did not notice any breakage during replacement, nothing had to be forced. I have no idea what to try.