User Profile Service failed the logon - FIX
Just in case anyone has had this error recently, I thought I'd share a fix we found. The issue is to do with clients not being able to logon and displaying this error...
We found various explanations online, but the most recent suggestions were that this was due to a January 2014 update. However, we recently upgraded IE in our suites to version 11, so who knows?
Anyway, the error can be fixed by altering the NTFS permissions on C:\users\default (You'll need to show all hidden drives in order to see this folder) and select the "Replace all child objects permissions with inheritable permissions from this object" so that it is enabled
This will allow users to login from then on. I can't explain why, as before making this change all subfolders and files have permissions that reflect the default user's.
Anyway this can be done through GPO, under Computer Configuration > Policies > Windows Settings > Security Settings > File System.
Hopefully this gets someone out of trouble!