I ran into something similar on Linux Mint. Never seen my installation kill itself before until this. Ended up booting into Recovery mode from the grub menu, and rolled back using Timeshift restore.

For me, the culprit was the ubuntu-drivers-common update, because after I rolled back, I was able to install all the other updates without issue. I just blacklisted this one update to keep it from showing until the next version is released.

According to the ionos api documentation, the API key is formatted as publicprefix.secret

Is that how you entered it in your config?

https://developer.hosting.ionos.com/docs/getstarted