I'm using SDDM (https://github.com/sddm/sddm) as my Display Manager on ArchLinux. The Qt4 build works fine, but the Qt5 build crashes.
On two of my computers, everything works fine, but on my NVIDIA Optimus Laptop, the SDDM greeter crashes when my display setup script is ran. I need this script+a modified xorg.conf to make use of the Nvidia card, otherwise the display will just stay blank (see here for more information https://wiki.archlinux.org/index.php/NVIDIA_Optimus#Using_nvidia). I've removed my optimus setup to try with only the intel card, and it worked fine without crashing.
I don't seem to be the only one with the same issue, although another user who reported it didn't mention an optimus setup (see https://github.com/sddm/sddm/issues/113).
I'm getting always getting the following error when the SDDM greeter starts (even though the script wasn't ran yet):
QXcbConnection: XCB error: 148 (Unknown), sequence: 154, resource id: 0, major code: 140 (Unknown), minor code: 20
Unfortunately ArchLinux doesn't provide debug packages, so my backtrace doesn't give the cpp files and the lines numbers, but it seems to be identical to the one reported on SDDM bug tracker. It's the one I attached, as it's more detailed. I could try to compile debug packages myself if it's really needed.