Lync 2010 Attendee certificate error

Hi,
I have an environment of group conversation and my external partner cannot sign in into conference call (outside my public domain name and Office 365 accounts) with Attendee client due error like that one image I post: Computer is Windows XP SP3, IE8, Silverlight 5 installed.
Can you please suggest how to resolve this issue?

Error : There was a problem verifying the certificate from the server. When contacting your support team, reference error ID 101 (source ID 238). Troubleshooting information is available online. Including best practices for using Lync.
