Typically, this problem is caused by something in the data in the fields which that portlet loads, being blocked by the SSO setup or security software or network appliance. Changing the portlet to NOT load data without filtering should resolve the issue in most cases. In this particular case, there was a field added to the portlet which contained links to SharePoint sites for Ideas. We simply removed that field from the portlet, and the problem was resolved.
Problematic URL example: https://projects.xxx.com/EUC00020-000/Project%20Documents/MS%20Office%20Upgrade%20-%