Icons missing from toolbar

Cognos Analytics 11.0.x
Microsoft Windows Server 2016 Standard

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:

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:

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.