Problems with booting the system

[Watermann]

First time this happens. At boot after a looooong time displaying a black screen, it suddenly displays the grub screen. I can select some possibilities but do nothing because I do not know what to select.
Then, again after a long time displaying a black screen, it suddenly boots and displays the login screen. Then after login everything seems to work normally.
I reinstalled a Timeshift backup from 4 days old version but that did not solve the problem.
Any suggestions what I can do to get things running normally again?

[Snowhog]

So the Grub Menu is displayed. You opt to do nothing; not an issue; and after the countdown timer value has reached zero, Grub boots to the defaulted option; normal. Then you experience a long delay on a black screen before being presented with the SDDM Login Screen. Correct?

[Snowhog]

it suddenly displays the grub screen

What do you mean here? The Grub Menu, or the Grub prompt?

[Watermann]

What do you mean here? The Grub Menu, or the Grub prompt?

I get a grub display where I can select things. If I do nothing (I do nothing) then it starts automatically and after a long time with again a black screen, I get finally the login screen.

[Snowhog]

So the Grub Menu is displayed. You opt to do nothing; not an issue; and after the countdown timer value has reached zero, Grub boots to the defaulted option; normal. Then you experience a long delay on a black screen before being presented with the SDDM Login Screen. Correct?

[Watermann]

So the Grub Menu is displayed. You opt to do nothing; not an issue; and after the countdown timer value has reached zero, Grub boots to the defaulted option; normal. Then you experience a long delay on a black screen before being presented with the SDDM Login Screen. Correct?

Yes, that is correct.

[Snowhog]

After reaching the Desktop, open Konsole and type: systemd-analyze blame > blame.txt

Attach the blame.txt file in your reply so we can look at what is causing the delay.

[Qqmike]

So, when you do finally get booted into your Kubuntu OS, and everything is "OK,"
you might try -- for the heck of it -- reinstalling grub and updating the grub boot menu:

Open Konsole, type
sudo grub-install
and press Return key;
and then after that finished, do:
sudo update-grub
and press Return key;
and re-boot.

Frankly, those long pauses where the screen is blank could indicate that Grub, or the system, is
having difficulty locating things (your ESP, your OS files, etc.), and/or something is damaged in the filesystem.