I found a solution for me. For me the problem was caused by a corroded contact of the haptic engine. The phone kept stuck in bootlop because of a broken hardware. After unplugging the haptic engine my phone instantly booted again. So just unplug every hardware component one by one. E.g. unplug the haptic engine, check if phone boots, if not, plug in the haptic engine again and try another hardware part. repeat the procedure until your phone boots. Maybe to speed this up, just unplug every hardware component and check them and their contact points if they are broken or something is unusual, maybe you can see something at first sight.