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This was a known issue, 418: Unknown Exception
In future, please check the Status page first, as well as search latest questions on the Make forum to see if others are also experiencing the same issue.
Additionally, for technical issues or bugs like this, directly contacting support can often lead to a faster resolution. They have access to your specific account details, scenario and scenario logs, server-side logs, and internal tools and resources, which allows them to investigate more thoroughly than what you have access to.
You can open a new ticket, or if you are unable to login for some reason, you also can reach support using the contact form on the website.