Finally, I think the "issue" is due to the fact I created this account as a Sandbox tester of my app, it is not an enrolled developper account, and such users can't access appstoreconnect.apple.com.

Thank you for your post @jargote, I asked to the account owner to agree terms, and I also agree with you : instead a blank page, we should have a more explicit message from Apple. I will tell you if you solved my problem, and made me save a lot of time too. Thanks.