That was a bug in a few specific versions of JFBConnect v4.x. To fix, you'll need to actually uninstall JFBConnect and then re-install it v5.0.0. All of your settings will be saved when you uninstall, but your plugins may be unpublished.
The problem was a bug in a few older versions of JFBConnect. Unfortunately, when you simply upgrade JFBConnect, the problem is not something that Joomla will let us fix automatically, which is why you have to uninstall first.
Hope that helps,
Alex
impossible to open ...updates/jfbconnect.xml
