First, try adding --log, like fdbbackup start --log -d blobstore://<api_key>:@s3.amazonaws.com:443/mytestbackup?bucket=<aws_s3_bucket_name>. You can read through the .xml file to find out exactly what’s going wrong.
In our case, there was a TLS issue and so we prefixed fdbbackup and backup_agent with FDB_TLS_VERIFY_PEERS=Check.Valid=0 FDB_TLS_CERTIFICATE_FILE="/etc/foundationdb/fdb.pem" FDB_TLS_KEY_FILE="/etc/foundationdb/private.key" FDB_TLS_CA_FILE="/etc/foundationdb/cert.crt". More information on that here.