I'm having the same issue Alex. The project I'm working on decided to use the webform as the primary way to get input back from the public. No changes were made to survey, and after the August 30 update, now all I get is a 'Failed to Submit' banner at the top. The survey works great in the iOS app and desktop (field app). Using the latest version 3.0.142, I created a newly published survey and again it still fails to submit using web form and works in iOS and desktop (field app).
I ALSO have related tables containing a repeat question.