@themustard, If it redirects you to the forum, you’re likely using an older version. Can you confirm? A few versions ago it was missing a constant from IPB’s class to resolve that issue.
There’s an important checklist to help troubleshoot.
What are your Friendly URL settings? (in 3.1.4 it’s located in Tools & Settings > System Settings > System > Search Engine Optimization)
Redirect to new friendly URL format? – Yes
URL Type – Query String
Use .htaccess mod_rewrite – Yes (and your .htaccess contains the settings it suggests)
The forum that you enabled for that category did you enable HTML posting in IP Board for it?
What about the member ID? Does it have permission to post HTML?