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Lync 2010 NTLM Client Authentication Mismatch

After installing Lync at one of our customers we experienced the following problem when the user was trying to log in.

When I try to make the remote sign-in with the appropriate credentials, Lync reports me “Cannot sign in to Lync. Lync was unable to sign in. Please verify your logon credentials. But it only happened on Windows XP/Windows 7 machines.

When I try to sign-in using the same credentials and Windows 2008 R2 as the client, Lync is signin in!

TheLync server was running Windows 2008 R2. With Windows 7 and Windows 2008 R2, 128-bit encryption is (by default) required for all NTLM authentication requests. After disabling the 128-bit encryption requirement using information from http://technet.microsoft.com/en-us/library/dd566199(WS.10).aspx and running a gpupdate the computer was able to authenticate successfully.


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