This error can occur if we have an event registration form part on the page that is not fully configured. To resolve the error, we can edit the event form part and correct any errors in the part editor.
  1. Navigate to Site Explorer > Pages & Templates.
  2. Search for the page that is returning the error.
  3. Select the page in the search results (do not click pencil to edit just highlight the correct page in search results).
  4. Under the page search results, the page content will display. Click the pencil icon to edit the event registration form on the page.
  5. Click to save the event form to check for any errors. 
  6. Correct any errors that may appear when saving the form and save the corrections.
You should now be able to successfully edit the event page.