This problem happens very often. I think that the “restart” (as you did) can take care of this issue.
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 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