Grant says, is the multi board warming up at all? Obviously it is within its ratings but if there is a slight fault within the board or one of the plugs plugged into the board, it could be heating the board slightly. If there is heating of the board, especially by the circuit breaker, it will tend to trip at a lower load than normal.
There could be slight corrosion on one of the plugs, or the contacts within the board itself may not be putting enough pressure on the plug pins, which could also be a cause of heating. I would not expect this with an HPM board but it could happen.