hello philipp.schwarz,
please perform a whole flash process to update board configuration settings.
BTW,
you should access Jetson AGX Xavier OEM Product Design Guide for reference,
Jetson AGX Xavier brings five UARTs out to the main connector, check pin description for the details.
you should also check this topic for the changes to disable combined uart on Xavier, Issue in disabling combined UART - #6 by JerryChang
thanks