I can log into AGOL assistent using my credentials for our portal. That works fine, but when I try to search Portal for ArcGIS, there is a timer-thing running in the upper right corner, but nothing happens. The same happens with "view My content" and "view my groups". Have tried it in IE and Chrome, same problem.
The timer indicates the page is still loading. Did you give it enough time to finish? Wait for a time out message.
Did you log in using HTTPS? If not, try that.
No or slow response can be many reasons. Assistant is a cloud based SAS. Could be a hiccup there, could be your end. Make sure you have other internet access.
Try it again later in the day?