This error can be caused by a bad preset. If this is the case, reboot the unit holding down the PROGRAM key at the second startup screen to load a harmless preset.
If this happens on internal sync and you are running the latest version (5.5) it will be a bug and we'd be grateful if you would tell us.
If it only happens on external sync (e.g. ADAT), it could be a bad source. Look at the sample rate indicator on the SETUP/clocks screen to see if it is stable.