403
error, does that apply to download through the file mount or through the S3Drive interface?
403
error indicate that there isn't read access available to the resource.
I assume that it worked fine initially and then stopped at some point without any changes to key permission / IAM, etc.?
For some providers it's returned when e.g. transfer quota was exceeded (e.g. Backblaze)
10GB
download a day. Mount can be data hungry, as if you open a folder, macOS will likely try to fetch some files in order to understand their format, create thumbnails etc.
I am not sure if that's the case on your end, but would be good to rule this out.
Backblaze says that quota resets at 12AM GMT each day: https://www.backblaze.com/docs/cloud-storage-data-caps-and-alerts
which implies that 20 minutes after you've posted your message it may well started to work again.