500 error thrown after signup form submission

First reported by an agency contact (Holly D.) signing up ~8 hours ago:

"I tried to sign up to enform but got two errors. I'm on my phone at the moment but can share screenshots if you're unaware of the issues to help identify the problem."

I've been able to reproduce the error by trying to signup via email from mobile and desktop. After submitting the signup form (https://app.enform.io/free-trial), an error message appears: "500 Internal Server Error
If you are the administrator of this website, then please read this web application's log file and/or the web server's log file to find out what went wrong." (https://app.enform.io/accounts).