The problem is caused by the Italic BBCode due to the conflict with the Underscore templates system BuddyPress uses. There are two ways to solve this:
- You can disable Italic BBCode (Forums -> Tools -> BBCodes -> Disable BBCodes) or,
- You can limit BBCodes to the bbPress content only (Forums -> Tools -> BBCodes -> bbPress Only). If you do this, BBCodes will work only with bbPress topics, replies and forums.