We had the same issue with a course that sprung up yesterday. In our case, teachers were unable to see the page load, although students could.
After many queries, log searches, and source code read-throughs, we concluded that a discussion within a forum had no discussion posts associated with it, throwing an error at line 136 of this file. After confirming this was definitely the case, we deleted that row in the forum_discussions table and the forum activity began working normally again.
Hope that helps someone down the line.