Received the strangest case on my desk today. A user was unable to sing in to the internal SharePoint site. When the user tried to logon via UAG everything worked fine.

On the SP server quickly found a lot of these events.

Account For Which Logon Failed:
  Security ID: NULL SID
  Account Name: Username
  Account Domain: Domain
Failure Information:
  Failure Reason: An Error occured during Logon.
  Status: 0xc000006d
  Sub Status: 0xc000006d

Binging that did not help much, so I turned to my Security log from the domain controllers, and found a much more helping event.

There it was, the event stated that the credentials was invalid.

The computer attempted to validate the credentials for an account.
  Authentication Package: WDigest
  Logon Account: Username
  Source Workstation: SomeServerName
  Error Code: 0xc000006a

Strange, as the user was both able to logon via UAG and start a VPN connection. Well, trust the DC I thought, and opened ADUC and reset the users password to the same password as before and guess what? It worked!!

How this is possible, I do not now, but if I find the time, I will for sure dig into it.