I was developing web apps when suddenly dolphin, yakuake and konsole started crashing upon opening.
After a couple of reboots I noticed some errors in journalctl

dic 20 17:19:04 raul-claro kwin_x11[6485]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 37779, resource id: 44040904, major code: 18 (ChangeProperty), minor code: 0
dic 20 17:19:04 raul-claro systemd-coredump[7734]: Process 7116 (konsole) of user 1000 dumped core.
dic 20 17:19:04 raul-claro kwin_x11[6485]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 37814, resource id: 109051906, major code: 18 (ChangeProperty), minor code: 0
dic 20 17:19:05 raul-claro systemd-coredump[7733]: Process 7152 (konsole) of user 1000 dumped core.

But also some video drivers errors (i guess)

dic 20 16:50:56 raul-claro kernel: nouveau 0000:01:00.0: bus: MMIO read of 00000000 FAULT at 10ac08 [ IBUS ]
dic 20 16:51:20 raul-claro kernel: nouveau 0000:01:00.0: bus: MMIO read of 00000000 FAULT at 6013d4 [ IBUS ]

I can open firefox and visual studio code for example, but dolphin/konsole/drkonqui fails to start.

The system is updated, and there were no updates today

Any help?

You use the nouveau driver? inxi -g
Then make sure to switch to XRender from OpenGL, see the ISO testing thread. Nouveau no longer functions with new Qt 5.12 OpenGL changes.

Just found out that all works too when removing the nouveau driver and just use the kernel modesetting.

I dind't know I used nouveau

inxi -G
Graphics:  Device-1: Intel HD Graphics 620 driver: i915 v: kernel 
           Device-2: NVIDIA GM107 [GeForce 940MX] driver: nouveau v: kernel 
           Display: server: X.org 1.20.3 driver: intel,nouveau unloaded: modesetting,vesa tty: 240x67 
           Message: Advanced graphics data unavailable in console. Try -G --display

I ran

sudo pacman -R xf86-video-nouveau

and rebooted, but the problem still persist.

How can I disable compositing without having access to System Preferences? I tried Control+Shift+F12 but it doesn't seems to work. EDIT: disabled in ~/.config/kwinrc , the problem still persists. Will try iso test

I tried a new user, the same results

False alarm, apparently the reason was that i was left out of space. 100% full, remove some cache and everything start working right away.

Should I keep nouveau?

Info is too sparse so far, but seeing this is a hybrid system, you might not use the nvidia card at all. But either way, personal choice whether to use the modesetting or nouveau driver.