Thank you so much for the help. This was the fix for me. I had the plasma 5 theme and needed the plasma 6 one. Also the last tip about themes is very appreciated and will be considered. <3

I’ve tried switching lock screen themes and gotten it to work temporarily but it goes back to default with that red error text on a reboot.

I have indeed tried that but it hasn’t worked.

I’m running Fedora 40 with kde plasma DE if it helps any