I have not personally seen that problem. My thought is this:
On your grub menu,, press "e" to edit, add "3" to the end of the "linux /boot/vmlinuz-5xxx" line, and then Ctrl-X to boot to tty 1. From there, if you log in as your user, then "startx && exit", it should generate error messages while it attempts to bring up sddm. I think you need those messages to resolve the problem.