I'm going to try to "connect" my AGOL login to MyESRI. I don't understand what MyESRI is or why it's involved in opening Pro, but it's the only place where the AGOL login isn't already connected.
Any idea whether this is the right move?
I do plan to call support next week if this latest idea fails.
--Didn't work. I connected the AGOL username to MyESRI, gave it all permissions, and tried again to start Pro. Still get the "username not a member of the organization" error message. I give up. Next stop ESRI support.