laptop authentication via Windows Database through ACS

Hi, We're getting a lot of "Machine authentication is not permitted" showing up in failed logs on the ACS server (v3.3). 2 laptops, that have previously connected successfully (WPA), and whose users are permitted via the Windows Domain Controllers, are side by side and within 10m (line of sight) of an AP. One connects the other doesn't, reporting back the above message. Cisco's only reference to this message is a table in a pdf listing the error message, but no explanation as in what circumstance does this message occur. Has anyone else had any exposure to this enigmatic error message, so they may shed some light my way??

The message is pretty self-explanatory. That particular machine is not permitted within AD to logon using machine authentication.