Although Lync is a great product, it can be slightly complicated, especially if you are a small/medium business and are NOT setting up 5 separate servers for Lync.
I ran in to an issue where two users could share content (screen or application) with each other without error. The moment a third user was added to the mix (i.e. Alice sharing content with Bill and Bob), I received "Screen sharing cannot connect to the server now". Somewhat confusing because screen sharing was working just fine between just two participants.
I do not claim to be a Lync expert. I am passing along information to help any Lync novices.
When two participants are connected, the session is peer-to-peer.
When three or more participants are connected, the Application Sharing feature depends on the Front End Server Multi Point Control Unit (MCU) to provide the sharing stream to all parties.
Error: Screen sharing cannot connect to the server now.
Lync Server 2010
Internal client to internal client(s)
Single server install
This error is commonly reported when the Application Sharing Server (ASMCU) service is not running on the Lync Server Front End pool. If the ASMCU service is not installed on the Lync Front End it is likely because the components have not been enabled within the Lync Topology.
Step 1
Verify Application Sharing service is enabled in topology builder.
If not, enable it and publish the topology. Rerun component setup on FE pool using the Lync Deployment Wizard.
Step 2
Verify "Lync Server Application Sharing" and "Lync Server Web Conferencing Services" are actively running on FE pool. If not, try to start them. Check Lync Application Event logs if start fails.
*In my case the "Lync Server Application Sharing service" was bombed. Starting the service resolved the issue. The service started successfully.
If the service fails to start, check event log and begin Lync debug logging (another topic).
Step 3
Verify whether the user has the appropriate conferencing policy applied and that the policy applied has EnableAppDesktopSharing = TRUE.
Get-CsConferencingPolicy
Good luck w/ Lync. Next steps: Lync migration to 2013 on-prem.
credit to NextHop/LyncMd
http://blogs.technet.com/b/nexthop/archive/2012/11/05/lyncmd-troubleshooting-lync-server-2010-application-sharing-internal-and-remote-scenarios.aspx
