No need to apologise for going to bed!
Developer tools in your browser are usually launched by either pressing F12 or Ctrl+Shift+i - this is what it looks like in Edge:
So what you need to before opening the developer tools is open the survey and complete it right up to the point before submission, then open the developer tools, then click submit. The Network and Console tabs on the dev tools will capture the activity that happened at the time of submission and may give some insight as to what went wrong.
You can save these logs by right clicking on any of the entries and choosing Save as...
Given that it looks like you've got the setup right, I'd also suggest raising this with your local Esri tech support - they'll be better placed to diagnose, confirm if this is a bug or known issue etc.