Product:
Cognos Analytics 11.0.x
Microsoft Windows Server 2016 Standard

Issue:
Users are prompted with login during there session with Cognos Analytics and some icons are missing from the main page.
SSO is setup and working, but you still get a windows login dialog now and then.
To ensure SSO is correct check this:
http://www-01.ibm.com/support/docview.wss?uid=swg21993357

Error message when you try to create a new report:
Error: Startup Request Failed: Webbegäran misslyckades.:401 – unauthorized
URL: ../images/common_icons.svg

Suggested Solution:
On the CA11 web server (gateway) go to the file manager and go to folder c:\Program Files\ibm\cognos\analytics\webcontent\bi\images
Right click on Images folder and select properties
Click on security tab
Ensure that the following groups at least are there:
SYSTEM
Administrators
Users

If a group is missing, add it.

In one case the group Users where missing, meaning that a person that did not belong to the Administrator group on the server got above issues when surfing to Cognos Analytics portal. Adding the local server Users group solved the problem.