Symptoms
You may see lots of Event ID 20029 and Event ID20028 errors that are sourced from Unified Communications Web API (UCWA) in the Event Viewer of the Skype for Business Front End servers. The errors are shown up repeatedly and could fill up with the hard disk of the Front End servers. After you apply this update, these events may still be raised, but at a much lower rate.
Resolution
To fix this issue, install the following updates:
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The July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Web Components Server.
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The July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Front End Server and Edge Server.