Over the past two days Epic Support staff have received a limited number of reports from our users experiencing issues loading UE4 (or our related products like the Epic Games Launcher) after updating to the latest Windows 10 Creators release (build 1703).
This issue does not appear to be widespread, and Epic’s QA tests on this same Windows version have turned up no issues. One user has reported that he was able to load the Launcher after performing a reinstall. However, if you are experiencing this issue after having recently updated to this Windows version, please let us know! We may have additional questions for you.