Hello penguin,
I just helped another old minetest friend with this issue Failure 418 at server announce.
He has two servers inside the same sub domain in a private address area (like me) but he has only one dyndns address what (i guess) is "connected" to the first server, and successful announced.
As he wanted to use the same dyndns address for the second server, he is getting an 418 Failure.
first to that:
HTTP-Statuscode
This is a list of Hypertext Transfer Protocol (HTTP) response status codes. Status codes are issued by a server in response to a browser's request made to the server. ...
So I guess our Client Minetestserver, asks via http at servers.minetest.net to register his server. 418 then tells fail.
Fail with code 418 means already that HAD a connection, but the server denys to register, whatever reason.
maybe problems :
servers.minetest.net is denying to register a 2nd or more servers on the same dyndns, even with different ports.
or
the 2nd server is not getting back the connection open to register the server, as the dyndns address is for the 1st server.
offered solutions to check out:
a 2nd dyndns address for the 2nd server and use this in minetest.conf (no other changes, just keep other Port)
and / or
check how the 2nd server is allowed to connect out (to announce and keep the port getting answers back.)
So at me, with 2 servers, both successful announcing servers have each an own dyndns address and there is NAT used, they can connect themselves successful to INTERNET, means they get back THERE own requests.
So far, this might be an hint what is wrong at you, and I will post the result, when I get the answer myself.