Error ID 28042 and UCMA sign-in fails in Skype for Business Server 2015

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Skype for Business Server 2015 More...Less

Symptoms

In a Microsoft Skype for Business Server 2015 environment, users can’t sign in to the Skype for Business on Mac or Skype for Business for iOS client. In addition, the following error message is logged:

HttpHeader:X-Ms-diagnostics 28042;source="ServerName.domain.com";reason="Too many outstanding requests for a single user."

Resolution

To fix this issue, install the following updates:

The July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Web Components Server.
The July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Core Components.

Workaround

To work around this issue, restart the UCWA application pool on the affected server or restart the affected server. But this issue will occur in one or two months.