Today I released a new update to the game, which fixes some issues with the server-client initial handshake (hopefully should also fix the bug where the users list sometimes isn’t visible when connecting).
Since the client-server connection mechanism is affected, all previous versions of the client won’t be able to connect to a new-version server.
To be able to connect again to the Poképon Center server, you must download the latest game release from here. Be aware that the new client version won’t be able to connect to older version servers as well.
Server maintainers are also invited to upgrade their server version.
Sorry for the bother, but it’s something that can happen for a game in development; as much as I try to avoid breaking releases, sometimes it can’t be helped.