Symptoms
Assume that the encryption for Skype for Business Server 2015 is disabled. When you try to join an Audio Video Multipoint Conferencing Unit (AVMCU) meeting, you may experience a delay or a failure.
Cause
Removing a muted user with disabled encryption can introduce the delay or failure in the AVMCU.
Resolution
To fix this issue, install the January 2018 cumulative update 6.0.9319.514 for Skype for Business Server 2015, Conferencing Server.