Does anybody know what the bug on the attached picture is and how it could be fixed?
This is a common graphics corruption issue that happens with some graphics card. I can reproduce such an issue reliably on a laptop with a broken Intel graphics adapter, so IMO this usually means that the graphics card need replacing.
See if you can clear the problem by restarting the Plasma desktop using the terminal command
killall plasmashell; kstart5 plasmashell. If it recovers, it could be a bug in Plasma but if it doesn't - it is definitely a hardware issue.