SOLUTION BELOW
I have never been in a more confusing situation regarding Linux.
I have a Dell XPS 15 9560, which had a dual boot Windows 10 / EndeavourOS setup. It was running fine for months. 10 days ago I updated Linux and after restart it couldn't boot anymore. It got stuck at "A start job is running for /dev/disk/by-uuid/..." (which is the root partition).
First, with the help of a friend of mine who is quite knowledgeable about Linux (he runs vanilla Arch, etc), we spent 5 hours trying to fix it but had no luck.
Then I decided to back up everything and do a fresh install. Aaaand the same error happened again on the first boot. Then I though "ok, probably some problem with Arch, lets try Fedora". Nope. Some similar error about not finding the root partition. (Here I must say that the kernel which was shipped with the ISO was working fine, but after updating to the latest one, it failed.) Here I thought "ok, then it might be a problem with the latest kernel, let's install EndeavourOS with the LTS kernel." Nope, LTS kernel also didn't boot. Then I tried Ubuntu and it worked, but that's not solving the problem. Then I decided to put another nvme drive in the laptop and try there. The same error again.
Now the greatest part: If I put the nvme drive into an external usb case, EndeavourOS installs, updates, boots without any problem, no sign of the error.
So now I don't know how to proceed... Maybe there is something wrong with the pcie port in my laptop, but except for the booting problem, windows is working, I can also mount and access every partition in the ssd through a live usb. So no other signs of problem with the port whatsoever.
I would be grateful for any advice as I've lost several days trying to solve this and I am out of ideas...
Solution: The last working kernels are from 11. August 2023 (both linux and linux-lts) linux-6.4.10.arch1-1 and linux-lts-6.1.45-1. You can download them from here: linux / linux-lts and install them with
sudo pacman -U the_path_to_the_package
Thank you all for the help!