Cannot save permissions to a workspace

I have a workspace in app.powerbi.com which I wish to add another user to as an Admin.  I've logged in then;

1. Click the three dots next to my work space in the workspaces menu on the left
2. Selected "Edit Workspace"
3. From the dialogue which appears on the right, I've added the email address of the extra user (this is an external user added to our AzureAD)
4. Save
5. Then Edit the workspace again, select the new person who is "Member" by default, and then changed his role to "Admin"
6. Save

From the chrome debugger you can see XHR requests returning a 500- after automatically retrying this request a bunch of times the system returns an error dialogue- the information for this is;