Restart dosn't work anymore

[Snowhog]

This is probably due to the source's mistake. How do I fix my system?

Redo the steps in post #3.

[kubicle]

Redo the steps in post #3.

Doing the full-upgrade is unlikely to help in the case that there are newer versions of packages installed from the oracular repos that are unavailable in the noble repos. To fix these, you would need to downgrade to the noble versions...and downgrading can be tricky.
I'd first assess the situation, to see how many packages are installed from oracular (don't need to sudo if you are in a root shell):
1.

(sudo) apt update

2.

(sudo) apt install aptitude

3.

aptitude search '~o'

Note the tilde '~' (not a dash '-')
(This will list installed packages that are no longer available from the repos, that is locally installed debs and packages from the disabled oracular repo...you can also view these in synaptic, but that won't help if you cannot reach GUI)

Downgrading packages can be done with aptitude in package manager mode "(sudo) aptitude", but aptitude has a bit of a learning curve to use properly, so depending on the situation it might be easier to reinstall noble or upgrade fully to oracular (which has the downside that you would have to upgrade several times to reach 26.04 LTS eventually).

But first I would check how many packages would need to be downgraded (steps 1 to 3).

[claydoh]

this can lead to version mismatches.

You mean "will"

Weird I didn't do that. Probably another installer bug.

Nope it is not a bug in the installer. Else everyone installing Kubuntu will be seeing this. Not happening.
Also it is specifically pointing to the main Czech server for oracular.

How do I fix my system?

No idea. We can't really say what that added repo installed before this was noticed. Roll back, if you can, to a point where that repo is not present and don't add it again?

[steve7233]

I will wait until next month when I get more money and get a new USB 3.0 drive and then copy my documents and downloads then just reinstall Kubuntu 24.04. That way I can be sure everything is working again.