I had the same problem, and I think it has to do with the way the controls are set. It happens when you change the answer to a parent item after you’ve answered to the child questions.
For example, if you answer that yes, the data is online, you get a new bunch of items activated. If you change that Yes to No or Unsure, the child items will be hidden but the answers will remain set. Than you get an error when submitting.
One workaround is to answer yes to everything, so you get all items activated, and start correcting the form bottom-up.
If that is too complicated, leave the page without submitting and start over (however, it may happen another bug, i.e. your form was mark as submitted, in which case you don’t get a second chance of re-submitting it).
I hope this is making some sense. I am not a technical persons, thus I don’t have all the vocabulary