Make sure you enter a FQDN, not just a hostname.
Also, that field is shared amongst all services. That's why you can only configure one single service at a time. If you need to support multiple services, they must all be configured in a script, and the DDNS setting be left on Custom.
It's not. It's simply not erasing existing nvram settings such as username/passwords, but those aren't used while the DDNS service is set to Custom. Only the FQDN hostname is used, to be "plugged" wherever the router generates an URL (for instance in an exported OpenVPN client config file).
I was using the full hostname (mythingy.crabdance.com for afraid & mythingy.servebeer.com for no-ip iirc) but still nada. Force refresh changed to 7 days, wildcard disabled host & ip validation enabled.
I did wonder if something was going wrong with those fields & the custom setting due to the Firefox popup as I said. However, with further testing, after leaving the "applying settings" screen going I manually ran the ddns-start script and got the "registration successful" message, and the "applying settings" message was gone! So some sort of issue with running the script maybe?
I was assuming that the router was using the hostname field to tell it what urls to use on the loop back; the fact that the no-ip DDNS doesn't work either suggests that there is something else amiss...