This is a well known issue that still has not been addressed by the author. I posted a fix long time ago on the forums. However, here it is again:
This has to do with how the user token is set. Even though you could have a user as a member of a group with access UAC is set up to not allow access remotely. To enable it, create the LocalAccountTokenFilterPolicy DWORD value at this key in the registry
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\system\
0 - build filtered token (Remote UAC enabled)
1 - build elevated token (Remote UAC disabled)
By setting the DWORD entry to 1, you will be able to access the system since the remote logon token will not be filtered.
This is discussed in this KB article,
http://support.microsoft.com/kb/947232. (It's for Vista but it applies to Windows Server 2008 R2)