Symptoms

Users can't access K2 Designer from an external Desktop.
The URl changes from www.XXXXXURL.com/designer to a much larger and extended URL which leads to a broken site.
If it was used in Chrome Incognito mode and the URl was changed then it by passes this issue.
 

Diagnoses

If it works with Incognito mode then it suggests that the external desktop is trying to connect via windows STS and it fails when using a normal browser. So this is clearly a permissions/authentication issue.
 

Resolution

Changing all authentication types from forms authentication to windows authentication.
Adding the URL and the FQDN(As a url) to the Designer >> System >> Control >> manage realms (also using forms and windows sts checked)