Could be a 'comms' issue 'tween tx and ATU. Many manufaturers use a multi-core as well as the RF coaxial feed. The multi-core can be a full comms path (send/receive) or a simpler relay selection signal with VSWR 'error feedback'. This is why we need to know make/manufacture as there can be multiple reaons for an 'error'. Using Occams Razor I agree with the above though - probably something as simple as the antenna wire fallen off the coupler!