Symptoms
Typically, this issue occurs in a large Microsoft Skype for Business Server 2015 environment.
Cause
This issue occurs because a certain task hits an exception that delays regular messages processing or leads to intermittent "503 Service Unavailable" error on Persistent Chat servers.
Resolution
To fix this issue, install the November 2016 cumulative update 6.0.9319.272 for Skype for Business Server 2015.
