Its not like this issue occurs every other day, but i see it once in a while on multi-user machines. I have never had the issue on my own single user machine. So it seems to me that the dangerzone lives on shared machines. I guess, that is nothing in windows that prevents a program from alter the user profile for other users, and that might be the root cause.
Sometimes a user profile just stops working. In those cases the solution is a simple as deleting the profile and log on again, as other has said in this thread.
In my current case i get a "Windows cannot copy file \\?\C:\Users\Default\ to location \\?\C:\Users\TEMP\. This error may be caused by network problems or insufficient security rights. DETAIL - Access is denied." when a new user tries to logon on. In this scenario, its most likely best to scratch the OS since some crutial permissions has been messed up. BTW google returns 14.400 results for that error message, and 550.000 results for "Windows cannot log you on because your profile cannot be loaded" so am i not alone in having this issue.
In the current case it is and 2012 AD controller and the errors has occurred right after trust was established with another domain.
My point is that there should be some tool that should be able to validate and repair the user profile and related settings/files/directories.
I guess there is a reason for the Win8 reset profile thing.