I am testing out your plugin and have just added it to one site on a multisite installation. I read that you haven't tested that yet, so may be why it's happening. I am using the latest version of your plugin.
I added a topic to a category and on homepage of forum it says 2 topics & 2 posts. But when I click on the category it says empty and when I go to dashboard to see posts it says 4 approved, but don't display the list there too. I have deactivated and reactivated the plugin a few times. Is there any other reason this may show that way? Are there know plugin conflicts?