Hi.
Freshly installed KDE neon 5.16.4 and I have issues with SDDM

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System:    Host: KDE Kernel: 5.0.0-25-generic x86_64 bits: 64 Desktop: KDE Plasma 5.16.4
           Distro: KDE neon User Edition 5.16
Graphics:  Card: Advanced Micro Devices [AMD/ATI] RV530 [Radeon X1600 XT/X1650 GTO]
           Display Server: x11 (X.Org 1.19.6 ) drivers: ati,radeon (unloaded: modesetting,fbdev,vesa)
           Resolution: 1280x1024@60.02hz
           OpenGL: renderer: ATI RV530 version: 2.1 Mesa 19.0.8

Screen flashes white during mouse movement over buttons like [Virtual keyboard], [Desktop session] and login / password entry fields.

You can see this bug on video:
https://youtu.be/vchlV2D7mYM

I'm new around here, but, the my first try would be turn off the dedicated graphics card and see if the problem is with the drivers compatibility. If yes, then try to change the driver and serach for a best solution for you.

If not, i unfortunately don't know how to help you. But hey, now you have already have a start to begin your little journey haha

English is not my native language, so, sorry any wrong words or mistakes.