Symptoms
This issue occurs when the HTTP stack on a Front End (FE) server is not working in a Microsoft Skype for Business Server 2015 environment.
In this situation, the following event is logged in the Skype for Business Server log on the FE server:
Cause
This issue occurs if the MCU Infrastructure message queue is full. In this situation, the UserPin service can't respond to any requests on the FE server.
Resolution
To fix this issue, install the March 2016 cumulative update 6.0.9319.235 for Skype for Business Server 2015, core components.
Workaround
To work around this issue, restart the RTCHost.exe process.
