It's an error you'll receive at the client side informing you that something went wrong with the web service call at the server side. To see more details about what went wrong, see the server log.
As noted by Alexander, this signals that a fault was received by the webservice caller (a toplevel xml element that's called "fault", to be exact). If you don't have an error log, server-side, at the same timestamp, I suggest you turn on tracelogging for webservices so that you can pinpoint which messages are triggering this exception.