This incident has been resolved.

Our team has pushed a fix on production, you can now access normally to Passport, we are still monitoring our servers to be sure everything is running smoothly

You may be having difficulties using our drag-and-drop email editor, Passport. Our engineers are investigating. We understand this is critical and are working to solve this urgently. We will update you here shortly.
If you have urgent questions, our customer support team is here to help. You can contact them here: https://www.mailjet.com/support/ticket/open