Error: The username you entered is not a member of this organization
When logging in to ArcGIS Online using the organization's username and password, the following error message is returned:
Error: The username you entered is not a member of this organization.
This issue occurs when a user attempts to log in as soon as a user from a different organization has logged out from the sign in page. The URL of the sign in page still contains the short URL of the previous organization logged in. So ArcGIS Online assumes the second username belongs to the same organization. The image below shows the short URL and the organization's name on the sign in page displayed after the user logged out.
Solution or Workaround
To resolve this issue, remove the short URL of the organization and change the URL to https://www.arcgis.com/home/signin.htm/. Press Enter. Log in using the username and the password of the required organization. The image below shows the URL of the sign in page and the Sign In text without an organization's name above the Username text box.