At this stage it remains unclear just many users are affected, but there is significant activity on social media and popular forums like Reddit. Additional issues include File Explorer crashes, (causing the entire explorer.exe process to lock), kicking users out of their Windows 10 user profiles and requiring the creation of a temporary user profile to get it back, and a black screen of death rather than the more typical blue.
05/27 Update: Windows Latest has published further information about the problems caused by KB4556799.
