IAS error message

I’m receiving the following error message when attempting to log-in to a wireless network using PEAP (see Ultimate guide):

“The message supplied for verification has been altered.”

This test box was working a month or two ago, but now is given errors when attempting to log in.

The certificate is still valid, and I’m not sure if anything else has changed. The IAS server is registered in the AD domain.

Any ideas? I can’t find any references to this error message anywhere!