Hi, since last Tumblweed update i have a similar problem.
I detected sddm is producing a crash in the system. So i started the system in init 3 (editing grub witn a 3 at the end of the init command) and ran:
‘update-alternatives --config default-displaymanager’ as root and chosing xdm als default display manager.
With gdm did not work, with gdm black screen as well, with sddm, after a crash, i could logon form terminal and start the X-session with startx.
As I said, for me now the system works with xdm als presentation-logon screen.
Waiting for a fix.
Best regads, hopefuly it helps