The KB4515384 patch was released to address an issue related to Cortana and the search box. It seems that following an update, the SearchUI.exe process was causing spikes in CPU usage.
While the patch does seem to have fixed the CPU spiking, it’s brought new issues for certain users. Some Redditors write that performing searches via the Start menu brings up a blank window, while others say a Critical Error warning is informing them that ‘Your Start menu isn’t working. We’ll try to fix it the next time you sign in.’