Exception: Malformed HTML content: <!DOCTYPE html><html><head><script>var I18n=JSON.parse(" 

The malformed HTML error likely happens when you copy-paste text from any HTML page into the question fields of your form. When the copied text containing invalid HTML formatting is pasted into the question field, the Google Script is unable to parse the tags and hence returns an error.

To resolve the issue, please open the Google Sheet associated with the form and open the Form Editor. Now edit the questions and type the text manually so that no invalid formatting is added to the questions.

Save the form again and it should resolve the issue.