Hi,
We've done some local testing and were able to replicate the bug. We also found that if you select a check box field type from the drop down, then press the cross next to the checked value field to remove its value, your able to add data to the form again (see attached image).
I've elevated this issue to one of our senior developers to look into.
Thanks for brining it to our attention.
Cheers,
Greg
Greg Thomas
PHP Programmer - interactivetools.com