ServletException error in Form

After creating a form and assigning some dropdowns for various fields, something strange happened ... it was all working one minute, then I was unable to edit the form anymore (the little pencil rectangle on the left wouldn't show up next to any of the fields) and all the fields which followed the one with the error disappeared from the view.

When you access the application using the main form, this error is shown in the last field displayed [a field called 'Type']:
<[ServletException in:/jsp/themes/common/form/formalone.jsp] null'

I've already taken the liberty of sharing this form with support[at]zohocreator[dot]com in case it's helpful to see.

Thanks for any help you can give. Zoho has a lot of amazing apps and I'm especially excited about Creator.