Page not found when trying to select or create an item for an array field

I’m running into an issue when editing or adding a content object in the webtop. When I first login, while adding/editing an item, I can create or select items for any array fields and am able to save my changes without error. However, within minutes, when I go to add/edit second or third item, I get a 404 error on the modal box that appears when clicking on the “create” or “select” button for an array field. I have to re-login to stop the 404 error. Any ideas why this occurs and how to stop 404s?