It happens because your e-mail has been reserved for AD-login by your AD-organisation. Therefore, you have to use your username, which is not your AD-mail to get access to external organisations.
Previously, you have used your AD-mail as username for all your organisations. However, your AD-organisation has now changed the login format to be an AD-login instead of a FirstAgenda-login - which means that in the future you can only use your AD-mail in your AD-organisation.
What should i do to get access to the external organisations?
If you do not have a username yet you need to get a username assigned from a administrator in your AD-organisation.
Please contact the administrator in one of the organisations you use. You find the contact mail by logging in to web.firstagenda.com and clicking on your profile picture in the top right corner.
As soon as you have been assigned a username, you can log in to the external organisation with your username and password as usual.