The ConfCheckMcuAvailability stored procedure times out. Additionally, you receive Lync Event ID 32050 that resembles the following in the Pool Member logs:
This issue occurs because of a deadlock in the ConfSetMcu stored procedure. When this deadlock occurs, Focus continuously sends an AddConference request to the unallocated multipoint control unit (MCU) in the front-end pool. However, the MCU rejects the request based on its current state.
To resolve this issue, install the July 2015 cumulative update 5.0.8308.920 for Lync Server 2013 (Front End Server and Edge Server).