Dear original poster, have you perchance tested with Cloudflare as a front-end proxy? It appears that the moment one activates the orange cloud icon, the site becomes inaccessible, presenting naught but a blank page.
The final conundrum I encountered was that the utilisation of Cloudflare resulted in a file upload limitation of 100MB. Should one wish to surpass this threshold, it becomes necessary to procure a Cloudflare subscription.
No, however, you could attempt to connect using the aws cli s3 command or a management tool like s3browser to verify if downloading and uploading function properly. Discourse rarely utilizes advanced S3 features.