There are a few people, me included that has the same issue in this forum.
Probably this comment is the most reasonable reason I know. (thanks for user Kannix)
We're having the exact same problem on several computers of our customers since friday.
On these systems we nailed it down to Microsoft Security Essentials as being the reason, most probably the latest engine security update.
It's not happening with other malware scanners for us.
It is most likely to happen on slower computers (no ssd) with automatic login.
Disabling Microsoft Security Essentials doesn't help, only uninstalling.
The issue with MSE locking ntuser.dat seems to be solved with definitions released around 5 pm (GMT+2) yesterday(4/9/2018).
We are no longer able to reproduce this issue on any of the affected systems.
Still haven't seen anything from Microsoft acknowledging this issue.
Edited by anton75, 10 April 2018 - 10:00 PM.