Sorry for the delay in responding to your other post, I overlooked the support request. I'll work with you in this thread instead of that one.
Honestly, I don't know the cause. We're leaving some pages open in Safari, Firefox, and Internet Explorer tonight and will see if we can recreate the issue. Can you answer a few questions for us to help us possibly narrow down the cause (in case we can't recreate it):
* Is there a particular browser your using, or have you observed this in multiple cases?
* Do you have to navigate to a new page for the error to appear, or does it show up on the page automatically?
* Does the error show up in a pop-up, or on the page itself?
Finally, we just added JFBConnect 3.0 Release Candidate 1 to our downloads area. This uses a whole new back-end connection to Facebook with their new interface. It's likely that this may fix the issue on it's own, but we'll try to investigate the problem both with 2.6.x and 3.0. If you'd like to give the new release a spin, we'd love to have your feedback!