As explained in the discussion https://moodle.org/mod/forum/discuss.php?d=390029 i think there is a typo in forum code, on page mod/forum/view.php, on line 97:

get_string('noviewdiscussionspermission', 'fourm'),

I think it's something incorrect and the last word should be 'forum'. I don't know which consequences it can have, but think it should be fixed.