OK, here's an annoying problem. I've added few people from external companies in my SfB client, and on my desktop machine everything works as expected. On the laptop however, I can see the availability info for only a single of those external user, all others show with the lovely questionmark icon.
Both the desktop PC and the laptop are using the same version on windows and Office. Both are using the same SfB user's credentials. Both are using the same network connectivity.