After upgrading my Synology from DSM 5.0 to 5.1 I noticed that Rsync from my Macbook to the Synology didn’t work anymore. Read my post on how to set Backup from Macbook to Synology using Rsync. To fix the issue was quite simple:

In the Synology DSM go to the config screen, choose Backup and Replication, Backup Services and enable Network backup service. I only have a Dutch screenshot:

Synology DiskStation Enable Network Backup Service

I’m not sure if I had this enabled on DSM 4 or 5 too and that it was disabled during upgrade to DSM 5.1 or it is a new setting.