It may also be worth noting that the C:\Users\Default folder is not the only one for which anecdotal evidence exists that, if missing from it's proper place, the update will fail. The lack of a ProgramData/Microsoft/Windows/Start Menu/System Tools has been mentioned as another
possible/probable culprit in the Answers Community threads "KB2919355 (Windows 8.1 Update) Fails on Windows 8.1 Pro with error Code 80070057." (pg. 2) and the now-locked "KB2919355 (Windows 8.1 Update) Fails (800f0092 and 80073712) on Windows 8.1 Pro w Media Center" (pg. 23, among others).