I have updated and cannot login to Manjaro anymore because it seems the theme was removed (I did not remove it).
How can I tell Manjaro to use another theme? I have no clue howto do that in the console.
Please remind me also howto get into the console on the login screen, because I forgot that already.
The theme has no bearing on whether you can log in or not. And in the absence of a custom theme, your login screen should default to the built-in theme.
Assuming that your display manager is sddm, you would set it in /etc/sddm.conf, or in a drop-in file in /etc/sddm.conf.d/. “Breeze” would be a safe choice.
CtrlAltF3.
The login screen is not the problem. Boot stops at the login screen telling the theme was deleted/is not available anymore thus refuses to let me login.
how would I know?
thanks 
sddm is still the default display manager for Plasma right now. However, plasma-login-manager is its announced successor as of Plasma 6.6.
It’s already available in Manjaro Stable right now, but unless you’ve manually installed it, you would still be on sddm.
Then look at the files I pointed you at earlier. The theme is listed in there somewhere. Just edit it and change it to Breeze.
Interestingly you seem to have been correct.
Upon restarting again it says now something different. The theme cannot be started because some plasma components are not there anymore.
So it seems the system upgrade deleted the plasma desktop somehow. I hope that does not mean that I have to reconfigure everything AGAIN.
Which one should I be using?
sudo pacman -S plasma kio-extras
sudo pacman -S kde-applications
or just
sudo pacman -S kdebase
If any of those are missing from your system, then you must have done something really stupid, because a system update would not be removing any of those packages. 
I would recommend installing plasma-desktop again. This should already pull in most of what you need.
Well, that depends on whether you’ve also wiped your home directory. That’s where user-specific configuration files live.
I have done nothing but booted in and let it update the packages in the app, not via Konsole.
The only thing that I didnt do was to use it for some months.
Not updating it on a regular basis caused a problem with the update once. Maybe thats what it is again.
Manjaro is a rolling-release distribution. It must be kept up-to-date, or else you will end up breaking your system — especially if you don’t know what you’re doing. And that’s what you did… 
I find it weird, that a prompted system update cannot do it’s update right though. Rolling release or not.
So what can I do, other than reinstall again?
To try to be constructive:
What is the error you see?
How do you know that your issue is caused by a theme not being present?
If the theme was from AUR (for example)
and it was not updated or is not available anymore or not compatible anymore
that is not a fault of “The Distribution” or whether it is “rolling” or not.
There is nothing weird about that. In a rolling-release distribution, version numbers and/or even filenames and directory contents will change over time. As such, there is no guarantee that it won’t break if you don’t keep your system up-to-date — actually, there’s more of a guarantee that it will break sooner or later, depending on how many updates you’ve skipped.
Manjaro tries making things easier for you by curating the updates and bundling them. But even then still, skipping too many updates will ultimately lead to breakage.
Plasma modules are missing.
Plasma.core among others.
sudo pacman -S plasma kio-extras did have no effect on that.
That’s why I told you to reinstall plasma-desktop. But if your system hasn’t been updated, then that’s going to lead to trouble anyway.
Try this… 
sudo pacman-mirrors -f && sudo pacman -Fyy && sudo pacman -Suu archlinux-keyring && sudo pacman -Syu kdebase plasma-desktop kde-applications
This says that kdebase cannot be found and that plasma-desktop is installed and will be reinstalled.
After that the plasma modules are still missing.
I get another error on boot, but it stops too briefly to get the whole thing.
error command loadenv.c check block-list (and something else but the message goes away so fast)I only mentioned kdebase because you yourself did. I did not check whether that package (still) exists.
Look, it is quite obvious that your system is broken, and we have no idea how or why, other than that you haven’t updated your system in a long while — probably from before the switch to where X11 support in Plasma became optional.
I’m going to give it one more shot, but if the following command does not fix things, then I’m afraid you’re just going to have to reinstall.
So here goes…
sudo pacman -S plasma-x11-session
Better cross your fingers… 
@Fred_VIE
Its important to give clear information also… When was the last time you updated your system.
There is no clean way to help you, when everything is in a dark dust.
/var/log/pacman.log
Should show you this.
Thanks, I am back in!
It is probably me for using Win for 30+ years but I kinda dont understand why the current state of an OS cannot be updated without it causing problems.
Likely because that never happened in Win.
I simply dont get the logic behind it.
Well since I have updated today that file would have said today, wouldnt it?
edit: I stand corrected, it was on Nov 26th.
Well, then as I thought, you had not updated your system anymore since we were still on Plasma 6.3. We went from there to Plasma 6.5, but as of Plasma 6.4, KDE decided to split off the X11 support in Plasma and make it optional, because as of Plasma 6.8, X11 support in Plasma will be completely discontinued.
Now, if I were you — I do not know whether you’ve got an Nvidia GPU — I’d try switching to Wayland at the login screen, given that X11 support is going to be dropped.
But your system was still on X11, and due to this having been split off, you could not log in again without the proper packages. Installing plasma-x11-session fixed that — again, for now, because Plasma users running on X11 are on borrowed time.
I’ve already explained it to you, in post #11.
MS-Windows is another beast altogether. For one, it is a point-release operating system, whereby an update only brings along only small patches which do not radically change any functionality. There are several GNU/Linux distributions like that as well.
In a curated rolling-release system like Manjaro, a bundled update means that there may be significant changes to the way core components of the system work together. In your case, there was a significant change in the way KDE Plasma works, and a move from upstream to Wayland, with X11 only becoming a still temporarily supported non-default option.
This then, compells me to post the following short essay, and please do read it, because it would appear that this applies to you.
I have used it for 3 years, but back then I could put the gaming Linux in the bootmanager. Since the last reinstall I have problems doing so and make it the primary boot option.
Due to that I have to go through the bios to switch systems which I am, it seems, too lazy to do.
In Cachy I failed to add Manjaro to the boot manager. I dont get why because I could add Cachy to the Manjaro bootmanager just fine. Just the choice which one to start I cant set, supposedly because of Wayland.