I'm creating a map, he logs in and says he only sees the splash screen, there are no menus to choose a castle, I see his messages in the chat, but he doesn't see mine, and when he tries to launch the game, both computers freeze.

Delete or move all maps located in the Heroes game's Maps folder, for example, to some subfolder.
But (!) leave at least one map in the Maps folder that can be played in multiplayer.

Do the same with the Games folder, where the game saves are stored. Leave up to 10-15 necessary saves in this folder, and delete or move the rest to another location for storage.

The fact is that often, due to a large number of maps or saves, the server player's computer cannot handle such a large amount of information (when reading the headers of maps or saves).