Check vboptions > message posting options > min chars per post
If that isn't it, create a new style with no parent - Styles & Templates > Style Manager > Add New Style > no parent - then browse the site using that totally default vbulletin style - do you still have the same problem?
If you are getting that message using the default style you just created according to my instructions above, then the security token error is from a modification. You should try disabling them one-by-one until you find which one is causing the error.