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?
I found it amusing when, in response to an issue, the admin asked someone to open a ticket by sending it to support@lemmy.world. At first I thought he meant to DM the account, but then I was like Ohhh, you mean actually send an e-mail to that address. I can totally see people confusing the two concepts.