Created attachment 129124 [details]
journalctl output with backtrace

kwin sometimes crashes on login after a reboot. The plasma desktop stays black. I have 2 monitors, the secondary one stays completele blank. The main monitor shows some of the windows of programs that are auto-started. I have attached some info I could extract from the system journal including the backtrace of kwin's coredump and some info from kscreen.
Restarting the xserver using <Ctrl-Alt-Backspace> (sometimes, multiple restarts are necessary) eventually leaves me with a working desktop. This may be due to plasma/kwin disabling compositing.
In the compositor settings, the followin info is shown:
> OpenGL compositing (the default) has crashed KWin in the past.
> This was most likely due to a driver bug.
> If you think that you have meanwhile upgraded to a stable driver, you can reset this protection but be aware that this might result in an immediate crash!
> Alternatively, you might want to use the XRender backend instead.

Re-enabling OpenGL detection does not result in an immediate crash. It works just fine.

STEPS TO REPRODUCE
Not entirely sure. The crash does not happen on every reboot, but only after booting the system. When only logging out and in again, I could not reproduce the issue. 
No crashes happen when the compositor is disabled.


SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.15.0
Kernel Version: 5.6.15-arch1-1
OS Type: 64-bit
Processors: 12 × AMD Ryzen 5 2600X Six-Core Processor
Memory: 31,4 GiB
GPU: NVIDIA GTX970

ADDITIONAL INFORMATION
Compositor settings:
Scaling method: precise (note: german setting "Genau")
Rendering Engine: OpenGL 3.1
Noise Prevention (vsync): Automatic
Keep the window thumbnails: Only for visible windows
Allow applications to block the composition: no/false
Theme: Breeze

Possibly related: bug 418951