Any custom task I design a form for in Nintex forms crashes, but the default Sharepoint layout is fine

I've created a custom content type from an Assign To-Do task in Workflow designer, I then add columns to it through Sharepoint site settings/custom content options. Finally I design a form using Nintex Forms, but once this is published I get Sharepoint foundation errors when opening a task to edit it. As soon as I delete the form, the task is fine.

We're using Forms 1.2.1.0 and Workflow 2.3.7.0