Thanks for the quick answer, I will reinstall. I will let you know if it worked.
Hello,
so, I’m pretty sure now that it must have something to do with the last official updates. Here’s what I have done:
Downloaded the latest Manjaro Plasma iso and reinstalled it. I replaced my existing manjaro installation and kept the home partition.
It all worked fine, now I installed kernel 5.7 and 5.8 and rebooted.
It starts without problems with kernel 5.8.
Now I have run sudo pacman-mirrors -c Germany and then sudo pacman -Syyu
Even after that, it still starts perfectly.
Now I ran sudo pacman -S --needed $( < packages.txt), it downloaded and installed about 4 GB (without error messages), now I restarted and have exactly the same problem again. Since there were over 800 packages he installed, I don’t know which package caused the error.
So I will have to do the whole thing again and then install everything separately. 
@inko I had the exact same problem as you, and I found the solution here: Black screen since the latest update (tty ok)
Then the problem aren’t the updates. There’s a package causing the issue. cat /var/log/pacman.log should show you which if those packages were installed fresh or were already installed.