Are you still getting the above error when opening your survey in the web app? If so can you please share a copy of your survey for further investigation. This issue may have been fixed already in a recent update to the website, so please try again.
As for modifying the end date, are you referring to the start and end question types. These are system fields that are automatically updated in the field and web app, they can not be manually updated via a survey by the end user. If you need to modify them you could try to edit them via AGO or Pro.