I just had the same issue. Have you fixed it yet? if so what was the trick?
So I finally restarted my computer and all is well.
This problem happens very often. I think that the “restart” (as you did) can take care of this issue.
Maybe you can check the connection with the item in the taskbar:
This worked for my user! The user had to double click it and it was able to create a connection. After that, the user restarted ArcMap and the Sign In button was available!
This also worked for my user, however the connection to the Portal had to be re-established in ArcGIS Administrator after the Test Connection was performed.
This has not helped. Is there a solution to this? I want to sign in to a different online account to republish those maps in our new portal. Need help on the correct method to this madness.
You have close out of ArcMap then to go to the program "ArcGIS Administrator" on your ArcMap machine and go to Advanced > Manage Portal Connections > Add > Connect > Save
Then open ArcMap again and you should have the option to sign into your Portal now under File > Sign In
When I try this it tells me that it cannot connect to https://www.arcgis.com/, though I am connected to the internet and the site loads when I type it into my internet browser. What could be causing this?