Oh boy. What confusion have I caused here?
My point was:
1. I can't create games that are visible to others (when using the "Create Game" button), although those games show up just fine on the fs2net website, and are counted properly in the lobby.
2. I can't run a standalone, with the same symptoms, i.e. the standalone is listed on the website, but isn't visible to anyone.
3. I can, however, hijack one of the standalones and MC a game there.
This is most definitely not good behaviour, as there is no indication given by the game as to why it behaves this way. I know that there are issues with the fs2net site that are being worked on, but this means that the entire multiplayer experience is not as smooth as it should be, IMHO.