You problem is very strange, never heard something before!
Do you have a second desktop environment installed, beside to plasma/kf5 (kde), like xfce, gnome ...?
The login-manager, called dm, is sddm and the window-manager (wm) is kwin_x11 or wayland?
Ah, btw, if you and others paste code or, for example, the output of inxi here.
Please use the code (#) tags like,
[code]
inxi -c0 -Sxx ( -c0 removes the color of the inxi output )
<output without the color tags >
[/code]
The result for example:
$ inxi -c0 -Sxx
System: Host: hhl Kernel: 4.18.7-towo.1-siduction-amd64 x86_64 bits: 64 compiler: gcc v: 7.3.0
Desktop: KDE Plasma 5.13.5 tk: Qt 5.11.1 wm: kwin_x11 dm: SDDM
Distro: siduction 15.1.0 Thunderstruck - kde - (201512041044) base: Debian GNU/Linux buster/sid