Hyperbackup looses connection

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My brother and I each have a NAS at home. We want to back these up for each other. Initially, I have both synchronized on the same network. Now the DS918+ is with him and my DS923+ with me. Both are accessible via Synology's DynDNS and port forwarding. The first 3 backups over the network went smoothly in both directions but since a few days his hyperbackup service shows my server is offline after 2-3 hours but still sends data (and my NAS receives them too). Another 30min later it aborts completely. An hour or so later my sever is shown as online again (without reboot etc.) and the backup can be started again. During the whole time both NAS are reachable via Quickconnect from the outside without interruption. I'm a bit perplexed right now. What could be the reason for this?