Unfortunately, I managed to put my printer in an unusable state. Wanted to go back to Marlin to check something, but couldn't because Repetier host would now receive different unknown commands, like "" and "?", and block communication completely. Tried to fix this by flashing also the bootloader (ATmege2560) - didn't help. Went back to Repetier firmware - now the BLtouch doesn't deploy when homing (yes, no PINDA anymore), ... :-(
Anyway, I couldn't find any sources of the USB bridge either - so far for Prusa & Open source. I just found the string in the firmware dump (attached). Also dumped the EEPROM. I have no idea what is going on; I suspect one of Prusa's numerous "Extrawürste".