Product:
Cognos Planning 10.1.1
Windows 2008 R2 server

Symptom:
Error message when starting up Cognos Analyst or Cognos Planning Administration Console.

Error message:
An error has occured while starting IBM Cognos Planning – Administrator.

Main form failed to initialize.

Error Details:
Could not logon. Please ensure you can access Cognos Connection using the Gateway URI specified in Cognos Configuration.

Possible Solution:

Change to not use FQDN in Cognos Configuration, and use only server name in environment tab.
(above will work if you are on the Cognos Planning server itself)

Change from http://servername.domain.org:80/ibmcognos to http://servername:80/ibmcognos
in cognos configuration on all existing places.

or add the Cognos URL FQDN to your Internet Explorer local intranet zone.

select internet options
select security
select local intranet
click on sites
click on advanced
enter the web site e.g. http://servername.domain.org/ibmcognos
click add
click close
click OK
click OK

Close Internet Explorer and test again to login.