Virgilio,
Haven't seen that issue before. If you disable the JFBCSystem or JFBCContent plugins, does that go away? There's nothing that JFBConnect does on every page to read an XML file, so not sure what we would be trying to read to cause that.
Is the message in one of Joomla's error boxes (the blue or red box inside your template for system messages), or is it just plain text showing up somewhere? Can you link to a page this is happening on so we can see ourselves?
Definitely confused, but we'll help get you going,
Alex