I cannot really offer any help, but maybe some logs might help out, like dmesg (after failing to start sddm or X) or "/var/log/Xorg.0.log".
Also, to find it is a particular hardware/driver, maybe some output of inxi (maybe "inxi -v7" or something) might help identify a common device/driver for those having problems.
(BTW, my updated aptosid system, as well as my updated siducion running in qemu/kvm VM, do not have that problem.)