Failed Namecheap DDNS updates don't cause an error in the GUI.
I recently tried to set up Namecheap DDNS and ran into a minor issue. When I added the client it appeared to succeed in the GUI, but the logs show that it actually failed. Here is the error (host and IP changed):
/services_dyndns_edit.php: phpDynDNS: updating cache file /conf/dyndns_wannamecheap'ddns.example.com'0.cache: 127.0.0.1 /services_dyndns_edit.php: phpDynDNS: (Error) No Records updated. A record not Found;
Subsequent DDNS update checks showed the following in the logs:
php-fpm: /rc.dyndns.update: phpDynDNS (ddns.example.com): No change in my IP address and/or 25 days has not passed. Not updating dynamic DNS entry.
Adding the client should probably fail if the initial DDNS update fails.
I haven't tested it thoroughly, but I think Namecheap changed the way DDNS works when they updated their dashboard a few months back. In the past I was able to use any hostname with my DDNS password and it "just worked". Now it looks like you might be required to manually create an A record prior to using it as a DDNS hostname. The above error may be a relatively new response from Namecheap.