Sorry for bringing back an old topic but I have almost the same problem and I'm not sure what's causing it.
The problem comes when I try to change the language of the forum, no matter if it's via the user panel or via the admin panel.
When I select anything different than the default "English", and then click the "Save changes" button it displays:
"Bad request. The link you followed is incorrect or outdated."
I suppose it has something to do with the fact that I installed my forum in /board and then moved it to /forum.
Everything else is working just fine. 
edit: version is 1.3.4