When we install ‘ArcGIS for Office’ (A4O), in the Excel Backstage (click on the File menu -> ArcGIS), by default (out of the box), the resource file is set to https://la.arcgis.com (ArcGIS.com), and resource files for A4O get fetched from the ArcGIS.com.
After trying all the different URLs - I feel the ‘ArcGIS for Office’ (A4O) configuration file for Excel corrupted.
All the Excel file configuration URL information gets cached in the ‘ArcGIS Map’ folder in this location - C:\Users\<user_name>\AppData\Roaming.Please remove the A4O cache folder and see whether it helps.
Here are the steps to remove the A4O cache from the machine -
1. Close all the open Excel on your machine.
2. GoTo ‘C:\Users\<user_name>\AppData\Roaming' Location on your machine. Delete ‘ArcGIS Maps’ folder (this will delete the A4O cache from your machine)
3. Open Excel -> select ‘ArcGIS’ menu -> select ‘Add Map’ , and see whether clearing the A4O cache helps, and the ‘Sign-in to ArcGIS’ windows gets opened.
ArcGIS for Excel Team