I seem to have the same problem (WHS2011 unable to configure with a 2710n), so if you could post any useful info you get from Draytek support I would appreciate it.
Thanks

I did post to DrayTek support only to receive a reply asking me to post somewhere else. I cannot follow that up because I am off on holiday (vacation) tomorrow for 5 weeks.

I did do some more testing of my own. I came across a Microsoft utility that tests router functionality.
http://www.microsoft.com/windows/using/tools/igd

One of the tests is UPnP support. I ran it on a number of my machines with XP, Vista and Windows 7 and they all passed the UPnP test but when I ran it on the Windows Home Server it failed that test. This suggests that the problem may not be with the 2710n but WHS.

This is the result of that test on the WHS.

Result:
The operating system is Windows XP
UPnP is enabled by the firewall
UPnP services are running
UPnP Discovery is enabled
Detecting Internet Gateway Devices on the network
1 IGD(s) found
IGD Device Information:
Manufacturer Name : Draytek
Model Name : Internet Connection Sharing
Model Number : 1.0
get_StaticPortMappingCollection() returned NULL IStaticPortMappingCollection
get_StaticPortMappingCollection() returned NULL IStaticPortMappingCollection
get_StaticPortMappingCollection() returned NULL IStaticPortMappingCollection
Failed to get port mapping from the IGD

See you in a few weeks.

Oh, I should say that I am using WHS version 1 (based on Windows server 2003) and not WHS2011 (Based on Windows server 2008). It is interesting that you are getting the same problem on the newer system.

Cheers