Sddm broken, possibly due to QT5 missing XCB

After upgrading my manjaro vm to latest stable, the system (guest) is broken. It boots, it shows the login screen briefly but then it disappears. So I cannot login.
What I found is that qt seems to be missing stuff.

So how do I proceed here?

I don´t have akonadi, nor do I have qt5-styleplugins. The only thing that is somewhat strange is python:

guest kernel is 5.10

EDIT 1: downgraded python-qt package with sudo pacman -Syyuu. This does not solve the problem however.

The host is running kernel 5.15

Currently running: 5.15.48-1-MANJARO (linux515)
The following kernels are installed in your system:
   * linux510
   * linux515

however, I dont have the headers:

$ pacman -Qq linux515-headers
error: package 'linux515-headers' was not found

Would I need those too?

Update, I have tried many things. I noticed several things:

  1. There seems to be a lot of memory corruption going on. When I drop into a tty (the graphical session is broken starting from the boot), I can dabble a bit there. But when I execute somethingb journalctl --boot=0 it sometimes happens that the whole guest freezes, including my host. Luckily, on my host I can also drop into tty and kill the vbox session.

  2. Also, executing a shell command prints something and ends in multiple promts lines, just like I entered return a few times after executing the command. Really unexplicable.

  3. Sometimes the system boots with a graphical login screen, but then turns black when I start typing.

  4. Also, one time it booted with a very weird looking login screen, like it has applied a really broken plasma theme, it allowed me to enter my pwd, but on pressing return it went into the abyss.

  5. sudo pacman -Syyu finds nothing to update.

  6. When I decided to reinstall everything with pacman -Qnq | pacman -S ,
    it finds lot of corrupt packages and invalid pgp signatures. I followed these steps to reset the keyring: Pacman troubleshooting - Manjaro.
    After that
    Then it still finds invalid signatures/invalid packages, that it offers me to delete. There seems to be no way of ending this madness. It looks like each time it randomly decides which of the next bunch of packages it wants to declare as invalid.

  1. Reinstalling everything again on the guest. Deleting all cached packages and sigs from the guest. Now it needs to download a lot.
    7.1 When the guest is still processing the guest reinstall, if I interact with the host system, I get a doubly linked list error in the guest. So my suspicion of memory corruption seems to be correct?
    7.2 When doing it again, the signature problems have disappeared and while doing the actual resintall of qt5-base I get zstd decompression failed, restored data doesn’t match checksum.

So I guess this is a high priority problem, where vm guest and vm host spill over into each other. I dont see any warning flags in latest stable release notes yet. Maybe we should roll this back @philm ?

set QT_DEBUG_PLUGINS=0 on your command line and then try to start the app again. Scroll through the error messages and look for any libraries that say they failed to load. Then try and find any package(s) that have those libraries.

For reference:
[qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found. | Qt Forum]

Thanks! The issue with sddm seems to be a bit misleading though, as virtualbox seems to be causing al weird issues by itself. Lots of memory corruption; the real issue seems to be that virtualbox is broken for the most recent kernels. I have an Intel 4820k, which seems to be on the list of processors that is affected.