Update from my side: I just found that while entering the internal hostname, in my case “raspberrypi2”, does not work, as shown in my comment above, the IP address (IPv4) instead (in my case 192.168.178.39) works just fine.
When I do a “ping raspberrypi2” from my machine, I get this:
PING raspberrypi2(2003:c1:13cb:4c00:e41d:2f49:2c72:e8a8 (2003:c1:13cb:4c00:e41d:2f49:2c72:e8a8)) 56 data bytes
Maybe it’s a problem with the IPv6 and that it’s resolved like that.
On the other hand, when I use the external hostname (like www.mydomain.com) that points to the very same raspberry pi via DynDNS, the connection works fine with the sync client as well. When I ping that domain, I get an IPv4 address back.