Considering the following scenario:
You deploy a Skype for Business Server 2015 Directory pool in your organization.
You install the March 2018 cumulative update 6.0.9319.516 of Skype for Business Server 2015 on this pool.
In this scenario, when you try to start the Directory pool by using the Start-CsPool cmdlet, it could fail and the RtcSrv service crashes on each of the servers of the pool during the startup process.
This issue occurs because the Director calling into CFabricService::ActiveSecondaryOnReplicationPumpComplete. This references g_spFocus that will always be null on the Director.
To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Front End Server and Edge Server.