We are currently evaluating this issue and will update this article when we have more information.

In the interim, disable CAPTCHA from the form:
  1. ​Navigate to Communities>Forms>Multiple Registration
  2. Hover over your form and select Website Info
  3. Click Step 2 Form Options
  4. Unmark the Box next to Enable CAPTCHA
  5. Click Next
  6. Click Finish
  7. Click Publish This Event Now