TLDR; If you have issues with a blank screen or screen flickering on an older installation (but newer version) of KDE, try changing your Global Theme (Application and Desktop settings).
I have an installation of openSUSE Tumbleweed with KDE that is almost 2 years old. It has gone through countless updates, mostly without a hitch. I noticed, beginning 3 or 4 months ago, sometimes I’d log in and I’d get screen flickering or tearing. If that didn’t happen, it may freeze with a blank screen and just the cursor visible.
I’ve changed dozens of settings over this time, trying to figure out what it could be, but nothing seemed to be the fix. I disabled things, removed extra things I didn’t really need, tried to simplify everything as much as possible. They would seemed to work for a day and then the next day it would come back.
The one thing I hadn’t tried during this time was to change my theme. I use the Breeze Dark theme, with custom panels and widgets. Nothing crazy, but time consuming enough I didn’t want to lose them. For about a month now, my widgets hadn’t been showing up, except on a fresh boot. On Sunday I turned my computer on and my widgets were back, but they were old ones I’d set up and (I thought) removed over a year ago.
It occurred to me the issues began around the time of the Plasma 5.25 update and now I was seeing that, maybe, some old configs were still hanging around. I went into KDE’s settings and looked at the Global Theme. It had a warning about resetting configs, which is what I thought maybe I needed. I changed my theme to Breeze Light, rebooted, changed my theme to Breeze Dark, rebooted. The next day I logged in and had no issues. Today I logged in and had no issues. Normally, when I’d think it was fixed, it would come back, but this time it seems to actually be fixed.