I got the same error, here is how I resolved it:
- Downloaded logs from AWS.
- Reviewed Nginx logs, no additional details as above.
- Reviewed node.js logs, AccessDenied AWS SDK permissions error.
- Checked the S3 bucket that AWS was trying to read from.
- Added additional bucket with read permission to correct server role.
Even though I was processing large files there were no other errors or settings I had to change once I corrected the missing S3 access.