User profile unable to login for first time
C:\Users\Default may be broken, and the user logging in may not have permissions to copy the files for their user profile. This is especially common when upgrading Exchange Server. Check by looking in Event Viewer source User Profile General for Warnings, Event ID 1509. You might see an event a description like this:
Windows cannot copy file \\?\C:\Users\Default\AppData\Local\Microsoft\Exchange Server\v15\Configuration15316_100.sqm to location \\?\C:\Users\TEMP\AppData\Local\Microsoft\Exchange Server\v15\Configuration15316_100.sqm. This error may be caused by network problems or insufficient security rights.
To fix this, just enable permission inheritance for the affected files.