[PROBLEM] Distribution upgrade to Buster fails

[Nachteule]

If upgrade fails, it is very important to figure out why it fails

You can find complete log in file /tmp/aptstatus

So I assume that your system stucked in the middle of the upgrade progress and this is the reason for a non functional system

I have 2 installations on an external usb disk, so this can not be the problem

And yes, backing up /home and recover it after fresh install would work