Microsoft Lync 2010 is not a version that can be used to sign in to the server

If a client reports us above issue, then check if that user has any client version policy

This confirms us that user is unable to login to his client as client version policy is blocking the user to login to Lync.

To fix the issue: 

Either you remove the client version policy which is blocking the client. 

(Or) Inform the user to upgrade the Lync Client