The above behavior only occurs when cold starting the device. Entering and leaving BIOS already circumvents the problem. But so does pressing
ctrl+alt+del when the grub menu is shown. Any other approach - like multiple combinations of options in BIOS - did not work for me.
Update: The device crashed when on duty and rebooted constantly with an uptime between 10 minutes and 3 hours. It's currently under investigation again, but seems stable without network/load. Memtest passes 4 rounds error-free.