The fact that the user is not staged might be the problem. Staged users are not checked to see if they have permission to post in the category, but registered users are. So, if the user is not in a group that has permission to reply to topics in the category, their reply will be rejected because they do not have permission. The same applies to new topics and the ‘create’ permission.
There is a topic about this problem here: Category not accepting "anonymous email" from known users