The issue has been fixed, you should be able now to design your campaigns without error, we are still monitoring the issue for few hours to be sure everything is now stable. If you have any question regarding this issue, you can open a ticket to our support team.
Posted Dec 08, 2017 - 10:42 UTC
You may currently face some errors when editing campaigns, our technical teams are currently working to resolve the problem. If you have any question about this issue, please open a ticket to our support team on https://www.mailjet.com/support/ticket