I may have encountered this issue today when I upgraded Excalibur on a couple of different laptops. I saw some eudev errors during boot, but I wasn't able to find any related info anywhere in /var/log. Is there somewhere else these errors might have been logged?

Anything that accessed the display backlight had stopped working: console blanking no longer turned off the backlight, and brightnessctl couldn't find the display. Also, I'm not sure if it was related, but X would no longer start with the modesetting driver. Oddly, it still worked with the Intel and nouveau drivers (on different machines.)

I went ahead and installed the usrmerge package. It failed on the first pass, but I got lucky, and only had to delete a few relatively unimportant files to get it to finish. Everything seemed to be working normally again on the next boot, with no errors reported.

Edited to add: I just ran usrmerge on the second laptop with zero errors, which was a pleasant surprise. The backlight is working again on this machine as well; however, a Minecraft server is still failing to start, so it appears I still have other issues. The Minecraft issue was due to a Java update, and unrelated to usrmerge. All is now working on this system as well.