http://uc-dude.com/2014/05/27/complete-guide-for-troubleshooting-lync-outlook-clients-integration/
In one of my cases, the solution was to make string of internalEWS empty in registry
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Lync\sip_address@domain.con\Autodiscovery/InternalEwsUrl
