I faced an issue with Lync Application Sharing and File Transfers, where on of the users was Internal and the other was external, the sharing failed with the following message (sharing failed to connect due to network issues. Try again later.) After investigation this was found to be caused by a firewall on the client, it was caused by Kaspersky Internet Security, so always make sure to turn off any firewall on the client computer.
I am blogging about Microsoft Cloud and Security Solutions