Thank you, Gane. It works well.
For others who, as I did, would take a second longer to understand, you have to manually edit the file sync.conf in the directory /usr/local/resiliosync/var (on Synology that is) and change the entry "listen":"[0.0.0.0]:28888" to "listen":"[::]:28888". Don't forget to shut it down beforehand.
BTW While you are at it and it's down, you might edit the file FileDelayConfig to your liking as well.