I'm using ArcMap 10.4.1. I can sign into AGOL through ArcMap but when I try to add a basemap / content from AGOL I receive the error message "An exception occured during a webclient request".
Some searching and it looks like it might be a C# error.
I recently updated from windows 8 to 8.1 but have not had to reinstall ArcMap (it just worked after the windows update).