When Sitefinity executes multiple requests with same session id, the asp.net only allows one request to get the session state, so the application displays this message “The request queue limit of the session is exceeded”. This error is displayed when navigating to Permissions Backend Page.

Solution: Open the Sitefinity web.config as Administrator, find the appSettings section and add the following setting:

<add key="aspnet:RequestQueueLimitPerSession" value="[int]"/>

Replace the "[int]" tag with the desired value. The int.MaxValue is 2147483647. For example,

<add key="aspnet:RequestQueueLimitPerSession" value="300"/>

For more information:
https://github.com/aspnet/AspNetSessionState  

https://github.com/microsoft/dotnet/blob/master/Documentation/compatibility/throttle-concurrent-requests-per-session.md

https://support.microsoft.com/en-gb/help/4014594/may-2017-description-of-the-quality-rollup-for-the-net-framework-3-5-f