Backtrace or it didn't happen ;-)

(DrKonqui should show up and allow you to select "developer information")

No DrKonqui showing up.

How do you determine the crash? Flicker and changing PID?

Let's assume KWin really crashes and something deep down is really broken (so that DrKonqui doesn't show up or crashes as well): Do you think you could gdb into kwin from VT1 and on the crash move to VT1 and dump a backtrace there?

$ echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope # this is to allow gdb to attach
$ gdb --pid `pidof kwin_x11` 2>&1 | tee kwin.gdb # copy all gdb output to kwin.gdb
> continue
[wait for the crash]
> bt
[hit enter until the end is reached]
> detach
> quit

All window decorations disappear. Remotely logging in via ssh shows no more kwin_x11 running.

Uhm, I can't seem to switch to any VT with Plasma5.

I'm sorry, I was mistaken. It does seem to switch, but that makes kwin_x11 crash instantly and somehow switch back to X.

Rather sounds as if X11 crashes (are you presented w/ a login dialog?) - otherwise there should be no autoswitching of VTs.

The better approach then is to do this via ssh (same process)

Nah, I just lose all my window decorations at that point and kwin_x11 isn't running anymore. Will do via SSH tonight.

Now, "of course" running kwin in GDB doesn't result in a single crash within the last 8 hours or so ;-(

I'm not sure if this issue still exists, but it disappeared when switching to the internal graphics card and getting rid of my nvidia card. Feel free to close it, as I'm unlikely to be able to provide more info at this point.