Product:
Cognos BI 10.2.1
Windows 2008 R2 server
Problem:
Sometimes when users run report or try to run a dashboard they get a blank screen or error message about DNS or no contact with server.
This is an environment where there are TWO Cognos gateways in front of the Cognos BI servers. Then there is a load balancer in front of the gateways, this should do that when user surf to the DNS alias for the gateway is routed to one of the Cognos BI gateways.
This to be able to handle more users than one Gateway can, and give some failover in case one of the Gateways brake down.
The error only comes when the users surf to the DNS alias for the gateway, if they instead surf to the servernname (in FQDN mode) for one of the gateway there is no issues. Surf like this http://servername.domain.com/ibmcognos
If FQDN works, then the DNS service is OK.

Error Message:
Internet Explorer cannot display the webpage
What you can try: Dianose Connection Problems
or
IBM Cognos Workspace: Unable to connect to server
or
This page can’t be displayed

Solution:
After some investigation it was found that on Cognos web servers we had http header size set to 16kB, while on load balancer it was just 4kB. It was increased to 32kB on load balancer. This solved the problem.