I filed a support ticket and was advised to unlink the app in the portal (at Environments) and redeploy, thus creating a new version of the container which should no longer have issue with the use of the AWS S3 file storage . THIS WILL DELETE ALL DATA. I had a backup, but since you can't upload and restore a backup in a free app, I had to import the database locally and create export/import functionality to get my data back in the new sandbox app. So beware! Create export/import tooling first if you want to keep your data. Mendix Support said it was a known issue and solved (this was September 16.), but I still had trouble generating files.
Quick and dirty workaround:
1. Upload the same code under a new app then deploy
2. Use URLRedirector widget to redirect users from original to new app