Okay, a friend and I both got the game through Steam, and want to play online through Ubi.com.

Our findings are:
If you alt-tab out while hosting a game, other players cannot join the game, it says that the host has cancelled it, alt-tabbing back in lets people join, and alt-tabbing back out when they're already in does not kick them. To the host it appears as though they join the game, and then quit.

If the host of the game moves or tries to build anything, the game breaks. The host's game appears frozen and no buttons are responsive.
The non-host player gets a message saying the host has been disconnected.

Both of us are playing in windowed mode. Attempting to launch the game without steam's shell over it doesn't work, we go and find the .exe to run the game and the updatelauncher.exe file simply does not work (gives a message saying it could not update version information, please reinstall the sample: missing URLs, missing version targets). And the H5_Game.exe launches itself through steam, even launching steam if it has to.

If anyone can help us solve these issues so we can play together, it would be greatly appreciated.

Oh, and since we're playing through Steam, we have version 3.1, and both of us are running windows XP with the ports properly forwarded (at least, I think they are, telling us which ports to forward would help us check, but we followed this guide at a port forwarding place).