To reproduce the bug yourself:
1) Log in as an admin
2) Create a new forum
3) Browse to the new forum, and click New Topic, Moderate, or Watch Forum
4) BUG: you will be taken to the forum listing for the 1st category (strange)
5) Go to Admin > Host Settings > Cache and clear the cache.
6) Now you can post!
Not sure what's causing this, but it seems cache-related. And, when this happens, the topics.ascx.cs NewTopic_Click event handler is never invoked. Strange?
Seems like something we should fix before beta.