Thanks for completing the survey @_JonT. I don't know why you are seeing the authentication error. We are still receiving the survey submissions, so the data are collected. The survey works even if you're not logged in, but we can see anonymous as well as logged-in submissions.
We'll investigate the error. If you (or anyone else reading this) see this again, we'd love it if you could copy/paste the error you see and share it.