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The amp is probably seeing a short circuit in the output power stage. An output device (fet?) has probably failed; the short circuit event on the rails is detected by the SMPS short circuit protect circuit and pulling the modulator off. Since the modulator is off, the AC online relay is now turned off, leaving the amp with no power, basically disconnected from the AC.
If an output device has failed, replace faulty devices and refurbish all the other output devices. Check and replace the heat detector foam if necessary. When testing output devices always have the heatsinks installed!

Alain