forum.freeciv.org

There is a nation that the capital is marked as (unknown) despite I had established an embassy, why?

Because you have not seen the city where their palace stays, at least since the time it has been put there. It's interesting that in fact the server sends to tge client city ids of small wonders of all nations, but unhacked clients don't show it, and you still might know not the city's name.

Have you recently visited the city where the palace of the nation with the "unknown" capital is located, as it might resolve the issue?