This happens because the LuduvoUriHandler doesn’t know where to pass launch arguments to. There is no client located in /Applications or ~/Applications (if you have that folder created), so the handler times out as a result.
You can work your way around this issue by going to the Luduvo files in the terminal. You’re going to want to locate the bundle folder. Once you’ve done that, open that directory in terminal and cdto usr/bin/. From there, you’ll see two binaries, LuduvoEditor and LuduvoGame. If you want to launch the editor, just run it like this: ./LuduvoEditor. If you want to launch the client, here’s what you’ll have to do instead.
On any Chromium based browser, (firefox oddly doesn’t capture the info you need), navigate to the page of the game you’re trying to play. Right click on the page and click Inspect. From there, switch to the Console tab. Click Join and IMMEDIATELY cancel. In the console, you should see an entry for Launched external handler for luduvo://. On that same entry, right click the link and press Copy Link Address. Go back to the Terminal. Type the command in like so: ./LuduvoGame <pastedlinkaddress>. You should be able to join a game.
This is a really janky solution, and is the fault of the Luduvo developers improperly packaging the game for macOS. Ideally, it would be packaged into an .app file like every other macOS application.