Sorry to bring back this old post but that is the closest post I have found with the exact same issue I have !
I lost my free dyndns account (as it seem I forgot to visit dyndns.org) so I tried to configure the dlinkddns service with a different email address than my now dead dyndns.org account.
The dlinkddns service is activated correctly with the right credential at https://www.dlinkddns.com/. There it seem to work ok and I can login/out correctly. I see the host name too and it is manually updated with the WAN adress correctly.
Now on my DIR-655 RevA1/A2 1.21, I enter all the information :
Server address: www.dlinkddns.com (selected from the dropdown menu)
host name : ****.dlinkddns.com
user or key : ******@hotmail.com
pass or key : *********
verify pass or key : *********
time out : 24
When I apply the changes, it seems it want to connect but a couple second after, it uncheck the dns service and everything comes greyed out.
What am I missing here ?