Consider the following scenario:
• You deploy two paired Front-End pools in a Microsoft Skype for Business Server 2015 environment.
• You create many conference directories.
• You start pool failover between the two paired Front-End pools.
• There are many meetings being scheduled during the pool failover.
In this scenario, the pool failover fails.
This issue occurs because a back-end SQL stored procedure takes more than one minute to sync the change of the conference directories.
To fix this issue, install the May 2017 cumulative update 6.0.9319.281 for Skype for Business Server 2015, Front End server and Edge server.