A protection mode different from the ones described in the manual is invoked when I switched the unit off and switched it on again immediately. Several LEDs are on (and don't flash). Is that a different mode?
If you power off and then back on again there is a slight chance that you will hit a certain point where the processor will lock-up. This is caused by the drop in voltage when you power off; if you then power back on just before the processor activates brown-out (internal reset due to too low rail voltage), the amp will lock-up. Solution: Power off and wait a bit longer before powering on again.