The error does seem to suggest that the account being used does not have the correct permissions. Could I just confirm whether the account has the following permissions for the associated bucket?
GetObject
PutObject
PutObjectTagging
Do you have any other projects currently uploading to S3 that manages to succeed uploading? Although unlikely, a proxy and or firewall may also be interfering if this is failing for everything.
Could you also attach the raw task logs from the deployment so that we can have a closer look, please?
I temporarily gave the account global admin permissions and still received the error. I tried this on the 2018.9.15 instance and it worked, so I think it was a combination of the version and incorrect permissions.