We're in Moodle 2.6.0 on a Windows of some sort (please don't groan).
The "Add a new discussion topic" button is not visible for Teachers. It is visible for students and admin types. It was visible a year ago because I have a teacher who posted a summary of all the students' discussions in Fall 2013 in a different course (both courses are clones of a template course). In a new course using forced separate groups, no teacher has the button visible. It is not in the html when I view source.
Although the default for Capabilities/mod/forum:startdiscussion is inherit (allow), I explicitly set it to allow, and nothing has changed.
The only "event" we've had was an attempt to do the security upgrade to 6.2.3 (I think it was) that failed, and we rolled back to 6.2.0.