Ahh, I misunderstood that it was the basic Joomla authentication that was failing. I thought you were saying the Facebook login was breaking. Well, glad to hear that part works
Unfortunately, I don't know why the Joomla authentication would be throwing that error. My best suggestions to try are:
* In the Joomla Global Config, increase the "Session Timeout" to something higher than it is; at least 15, but possibly up to 60 or 120 (minutes)
* Try switching your template to something else to see if that has any effect
* Make sure the System - Cache plugin is disabled
* Check that you're running the latest version of Joomla (2.5.8, I believe).
Beyond that, I'm honestly not sure what to suggest. I don't think it's a JFBConnect issue at all. We'll gladly help how we can, but just not sure what it could be..
Alex