In Microsoft Skype for Business Server 2015, consider the following scenario:
You create a UCMA application that connects to a conference.
The UCMA application runs for some time.
The network connection from the UCMA application to the server is prematurely disconnected.
In this situation, the SUBSCRIBE session dialog is ended unexpectedly and you can't receive notification messages from the conference subscription. In addition, the attempts to reconnect the conference subscription also fail. Future updates on the SUBSCRIBE session dialog will fail to arrive to the UCMA application until the INVITE session is refreshed.
Logic to reestablish a failed connection was missing from the conference subscription dialog, and therefore it would remain broken until the invite session dialog was refreshed.
To fix this issue, install the July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Web Components Server.