I had an issue where the plugin I authored was having this issue with Divi builder. Thanks to Philipp’s comment above, I realized the issue was that my webpack build js library was conflicting, and the solution was to specify a non-default name for the webpackJsonp function. https://webpack.js.org/configuration/output/#output-jsonpfunction.Thanks!
Thanks for posting your solution. I had tried to give the s3 user only permissions it should need for backwpup and had similar issues. It would be nice if they provided a recommended IAM policy.