If you are having similar problems, let me know and include the body of text that you're trying to post so I can isolate the issue.
New Details: This issue affects private messages in addition to replies and new topics. It happens whenever the following BBcode is used four times or more [url=http://website]text[/url]. It has been confirmed with registered members.
I installed a vanilla version of the forum software to test if the issue was due to any add ons or themes, but the problem's still there. So, it looks like it might be a server issue.