For the record: This happens when you incorrectly use the two different domains; ffforums.net and finalfantasyforums.net

Basically, vBulletin has a protection system in place that says that you aren't allowed to submit a form (like posting a post, changing your mood, managing your buddy list, sending a PM) from one domain and then have it alter the database on another.

What happened was that you used say ffforums.net, but the script was coded incorrectly and it assumed you were using the finalfantasyforums.net domain, and thus this protection kicked in.

Since most decent programmers do not use absolute urls for their forms, the cause of this is the vBSEO modification installed by this forum.
Despite the fact that it has relatively no use, it will never be removed and thus it will never be fixed.

To be sure this does not happen in the future, stick to using www.finalfantasyforums.net to access the site.