Blank screen on boot after update because kwin crashed. Here's the mini stack trace from the journal, I'll try and get a proper one soon:

                                              Stack trace of thread 867:
                                              #0  0x00007ff868282c2c __pthread_kill_implementation (libc.so.6 + 0x73c2c)
                                              #1  0x00007ff868228abe raise (libc.so.6 + 0x19abe)
                                              #2  0x00007ff8682106d0 abort (libc.so.6 + 0x16d0)
                                              #3  0x00007ff86821173b __libc_message_impl.cold (libc.so.6 + 0x273b)
                                              #4  0x00007ff86828cd65 malloc_printerr (libc.so.6 + 0x7dd65)
                                              #5  0x00007ff868292074 free (libc.so.6 + 0x83074)
                                              #6  0x00007ff8690e3370 _ZL13init_platformRK7QStringS1_S1_RiPPc (libQt6Gui.so.6 + 0xe3370)
                                              #7  0x00007ff8690e628b _ZN22QGuiApplicationPrivate25createPlatformIntegrationEv (libQt6Gui.so.6 + 0xe628b)
                                              #8  0x00007ff8690e6e88 _ZN22QGuiApplicationPrivate21createEventDispatcherEv (libQt6Gui.so.6 + 0xe6e88)
                                              #9  0x00007ff8689027ad _ZN23QCoreApplicationPrivate4initEv (libQt6Core.so.6 + 0x1027ad)
                                              #10 0x00007ff8690eb48d _ZN22QGuiApplicationPrivate4initEv (libQt6Gui.so.6 + 0xeb48d)
                                              #11 0x00007ff869c43aa5 _ZN19QApplicationPrivate4initEv (libQt6Widgets.so.6 + 0x43aa5)
                                              #12 0x00007ff86b76c9e7 _ZN4KWin11ApplicationC1ERiPPc (libkwin.so.6 + 0x16c9e7)
                                              #13 0x000055ab10c19851 main (/usr/bin/kwin_wayland + 0xd851)
                                              #14 0x00007ff8682125b5 __libc_start_call_main (libc.so.6 + 0x35b5)
                                              #15 0x00007ff868212668 __libc_start_main@@GLIBC_2.34 (libc.so.6 + 0x3668)
                                              #16 0x000055ab10c21c55 _start (/usr/bin/kwin_wayland + 0x15c55)

It looks like there's an error "free(): invalid pointer" associated with the crash.