Zyxel NAS 542 with Bootloop after Installing Debian-Linux with OMV

I have a Debian Linux installed on the NAS 542. While writing the kernel there was a message that there was not enough space on the drive. Then I send the command that the next start should take place with the Zyxel system.

Now the NAS is in a boot loop, it starts with all LEDs in white, speaks to the USB stick very briefly and then starts again in a loop. A rescue stick is not loaded.

I assume that the system is missing the files it needs to start up. Can you do something there, maybe via the serial interface? A cable and a Linux Ubuntu 18.04 are available.

Thank you so much if anyone here could help.