In a Microsoft Skype for Business Server 2015 environment, you receive the following error message in the user's Skype for business App log of a UCWA client (Skype for Business on Mac, Skype for Business for Android, or Skype for Business for iOS) or in the WebInfrastructure CLS component of the server logs:
This issue prevents the user from signing in to the UCWA clients.X-Ms-diagnostics 28042; reason="Too many outstanding requests for a single user.
This error message can occur because of several causes that lead to the user having 40 outstanding requests with the server. Our analysis of the issue points to the pending connection requests between Skype for Business and Exchange Servers being one of the causes. You can further check for EventID: 32019 in the Application event logs, and run the Test-CsExStorageConnectivity synthetic transition to confirm that the connectivity exists between Skype for Business and Exchange Servers.
We have introduced a design change to time out the pending connections instead of accumulating outstanding requests for a user. To get this fix, install the July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Core Components and the July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime. If this issue continues to occur after you apply the update, we encourage you to contact Microsoft support, and examine the output of Test-CsExStorageConnectivity synthetic transition and Debug-CsStorageServiceFailures synthetic transition for further troubleshooting on this issue.
To work around this issue, you can restart the Front-End servers or reset the Internet Information Services (IIS).