I've tried other solutions such as deleting registry for AGOL, and also adding AGOL domains to Internet Explorer's trusted sites. None of this was a reliable solution as they only worked at times. I've tried connecting to AGOL via https and this has been consistently working for me. Open ArcGIS Adminostrator > Advanced > Manage Portal Connections. You'll see http://www.arcgis.com in the list. Click on Add and enter https://www.arcgis.com/ and then Connect.
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