When copying the link to another browser (internet explorer), I got this error message *“Oops, this form doesn’t exist (anymore). Most likely the owner of the form has deleted it, archived it, or disabled it. Please contact the form’s owner to confirm. Detailed error: Form with ID aQnxeCxtRjrfHFh8nx5W8z not found in /formList”
Note: when using chrome browser, it works with out any problems.
Looking at the link you provided I see your ACCOUNT SETTINGS has been checked with Require authentication to see forms and submit data. This is the place from where you log-out. Un-check the same and you should not see the login error message.