Issue with backing up QNAP to Synology NAS - any help ideas

I'm running a QNAP TS251+ and having run out of external USB capacity I have a new Synology DS218J NAS bought as a simple back-up for the QNAP.

I can access both NAS's on the network by typing in the IP address to the search engine.

Yesterday I managed to back-up from the Synology to the QNAP via rsync but the QNAP for whatever reason will not back up to the Synology. 

Using the QNAP Hybrid Backup app I get an error message - which seems to encompass just about everything:-

"Invalid port number or service is disabled. Please check your server settings. Please check your network settings, account and firewall, default gateway, and whether the detination host is behind a firewall"

I'm looking to do a simple back-up from the QNAP to the Synology. Any pointers as to where I might look to resolve this issue would be as always gratefully received.

Thanks

Andy