Consider the following scenario:
You apply May 2017 cumulative update for Microsoft Skype for Business Server 2015.
Some Video-based Screen Sharing (VbSS) supported clients and legacy clients are in a Skype for Business meeting.
In this scenario, you might be unable to join the application sharing session of the meeting.
This issue occurs because Skype for Business Application Sharing Server (ASMCU) doesn't include the rtcp-mux attribute in the Session Description Protocol (SDP) after the application sharing session is failed from VbSS to the remote desktop protocol (RDP) session.
To fix this issue, install the December 2017 cumulative update 6.0.9319.510 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime.