I have made multiple solutions with Ninitex Forms and Workflow for Office 365 and this is the first time I have run into this.
I originally ran into this error with a Personal Time Off request using a SharePoint list. I couldn't find the answer so I just deleted everything and started over. It all worked great for a few days, but today I started getting the error again. This makes it so no one can actually submit a request with the form I have created. The error seems to happen after I have made many design changes to the form.
I've attached a screen clipping of the error and a screen clipping of the form in the designer just incase that is helpful.