Can't create new topic in forum as the instructors but can create comments in the group. Student in course can't ask questions or place comments in forum. This was an issue raised a few weeks ago and looks that it was fixed but now we are back to square one. See attachment of error received.
Please check the gruop settings that forum is connetced to the group or you have select "Yes I want this group to have a forum " .
Also refer :
http://premium.wpmudev.org/blog/daily-tip-fix-for-buddypress-group-forums-error-when-creating-a-new-topic/