External users were receiving the following error in their Lync 2010 client which was affecting Outlook conversation history, searching for Outlook contacts and Exchange UM functionality:
Some external users were prompted for authentication after they had signed into Lync, cancelling the login prompt would present the error above but the user would stay signed into Lync. No errors would appear if the correct credentials were entered:
This pointed us to the root cause being authentication. Exchange Web Services (EWS) is published via UAG and on the server we could see Event ID 24 appearing frequently in the Application Log:
"The request from user at source IP address *.*.*.* to trunk intapps; Secure=1 failed because the request was unable to reply to an HTTP 401 request from application EWS. The session ID is…"
The issue was resolved by changing the authentication method from Kerberos to Both: