That can happen if you make a major change such as major update to the binaries (ie a software update) while the screen is locked. Or because the screen "saver" / "locker" crashed.
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
I didn't make anything except run my usual update like every time from the desktop, not when the screen is locked. Anyway to fix this?
Reboot after updates or at least log out and back in.
But I've also had this happen with nvidia graphics. They were just unstable after a standby.
Occasionally I get this on wake from suspend, doesn't matter if amd or nvidia system.
I just think of it as an annoying kde bug. It's harmless though.
You either reboot or just switch to another tty and unlock the session.
But the others are right, you increase chances of this happening if you updated the kde stack and didn't reboot/relog.
I've read that it was fixed in 6.1.1. I haven't gotten it since. Fingers crossed it's gone