Not related to AX, but since most of the times we are using RDP connections, this can be useful (I know I used it at least few times). Sometime you can get into problems with the user's profile and, at that point, the user will login with a temporary profile, which means no documents, settings, etc.
To fix this, just look at the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\ CurrentVersion\ProfileList registry key and find the key for that particular user by looking at all the keys / ProfileImagePath value which should match the path C:\Users\user_to_fix_alias. Export it (to have a backup, always do this) and delete it. Then, go to C:\Users and delete the folder as well. Note: do this from another user account, not the one you are just trying to fix.
Your user should be able to login again just fine.