MacMini 2018 bootloop (I tried everything...?)

Hi Everyone, I have a 2018 MacMini. It was in daily use. I used it with an eGPU and booted from an external SSD, but there was also a system (earlier OSx) on the internal SSD, which also worked flawlessly. I decided to sell the machine, so I disconnected the external stuff from it and entered the system. Driven by a sudden idea - instead of setting it up as a new Mac - I created a new user (as admin, no apple ID) and deleted mine. After that it went into bootloop. I thought "no problem, I'll reinstall". My first thought was Internet Recovery. There I received an error code -1008F, based on this, the machine has been locked. To remove Activation Lock, I read that I have to remove it from my iCloud account. After doing this, there was still no change. If I try to install anything (including Windows), the system restarts after the apple logo. The boot menu comes in, but whatever device I choose to boot (either the old one on which I used daily before this situation ) or a new installer, the same thing happens. After PRam reset, there is welcome sound, but then I don't even get to the boot menu, it restarts immediately. Next time, no sound, just apple logo and restart over and over again. At this point I don't know what else can I try. Already tried local recovery and safe mode, but neither of these working. Any help or idea is appreciated :) Thanks!