Hi people.
I had the same problem.
In my case I set up the forum by group.
My solution is:
1 - Go to users
2 - Go to groups
3 - Go to add a group
4 - After added a group I added a user to the group.
Moodle works like that:
If you set up the forum by group you must set the group and user.
All users that I created without set up on user/groups/add group/add user to the group DOSEN´T WORK.
All users that I did what I "sad" in this post works.
In my case solve the problem.
Sorry for the bad english