It's recommended to keep the UI at 1080p, and let the TV do the upscaling. Kodi's skin isn't supporting 4k (yet) anyway.
I'll try that. I'll also try reinstalling 9.2 on another drive to see if it is working fine. I've had some weird behavior recently with my nvidia-based linux workstations when back from sleep. It might be an nVidia-driver related issue as well.