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."
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.