that finally worked! Reader restarted and came back to normal mode
UsbOnlyReqProc() Sequence Error was still there so maybe that doesn't necessarily mean, that the communication between PC and Reader was broken.
I didn't have much time to start playing with Flashing again (hazardous soul), but UF2.1 gave me same error as the previous version. I'm going to go through all previous posts regarding your newest tool and will try again. Thanks!!