I have an idea of what is going on

Just say you start with Austria and get 5 land trade routes at the beginning of the game. these 5 land trade routes take up a spot in your trade route

So you develop a port and have 2 trade routes available but you do not have any open ports so you can't trade because you have 5 used but only 2 available

Now you build 2 more ports and have 6 available so you can finally open 1 sea trade route.

I think a land route doesn't require an open trade route but will use you your current and future trade routes, so you can't open sea trade route until you have more trade routs open then used


Bug or feature is hard to tell. May be a feature as to stop the initially landlocked empires from becoming super trading hubs


Another hard to notice feature is even if they are connected by land they also have to be connected by road so as United provinces owning Netherlands and Hanover are land connected but not road connected so a trade port in Hangover will not increase your trade routes until you own the province to the south of Hangover connecting them via road