Are there any sections of your forums that are for registered users only? In the 3.x series of JFBConnect, if you have it set to leave the user on the current page when logging out, and it happens to be a registered only page, in some cases you can get the redirect error. In 4.0 (coming next week), we have a fix for this.
If it's happening at times other than logging out, please try to explain more if there's any common steps (being logged in, not logged in, navigating to certain pages, etc) that may cause this to happen more often. That will help us figure it out more.
Finally, what version of Kunena are you using and what SEF extension are you using (or is that Joomla's default SEF)? Those answer may also help us narrow it down.
Thanks,
Alex