TS-410 is inaccessible and red blinking system led

[AutomatedChaos]

On my QNAP TS-410, the system led is blinking once in 0.5 seconds. The leds on the three disks are green and there is activity (they are blinking vigerously). I want to get in the system to do recovery, but I can not access the system. It can no be found on my local network, it is not on the regular static IP address (port 8080 or SSH) and the MAC address can not be found in the DHCP table.
QFinder also reports no NAS found.

Does anyone has a hint how I can access my system again?