Is one of your monitors HDR? I’ve been getting similar issues (EndevourOS) ever since upgrading to 555, where both of my monitors are black/no signal when logging into Wayland. Unplugging my primary monitor fixes the secondary monitor, and it seems like HDR is the problem in my case - bugs.kde.org/show_bug.cgi?id=488941
After deleting ~/.config/kwinoutputconfig.json, I could login. I did some tests and the issue happens when HDR is enabled. In the config file that put “highDynamicRange” and “wideColorGamut” to “true”. Just changing “highDynamicRange” to “false” doesn’t work, but if I put both to “false”, I can login again.
Simply deleting kwinoutputconfig didn’t fix the problem for me but I haven’t tried setting it as read-only yet (Probably a good idea to back it up first). My X11 is working normally so I’ll stick with that until this bug is resolved.