We have some really long running queries in our SSRS SharePoint integrated mode environment. After a few minutes of processing, we would see the following error:
Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 12019
Interestingly, this issue only occurs when using IE. In Chrome, the report keeps running and works fine.
In our environment, we have an F5 BigIP as the load balancer in front of SharePoint. All we needed to do was set the Idle Timeout to 3600 (default was 300) on the TCP profile for the virtual server.
In other load balancing solutions, there are likely similar settings.