Hi, realize this is old, but I wanted to reply. I think I have solved the same problem. I have been reading of the same problem around the net.
I had an idea!
I wanted to send the factory default sysex file to the mpx100 to see if it would help reset it to factory default. I use a program called MIDI-OX to manage system exclusive data. I hoped to find/download the factory default data and input to mpx100. Ended trying the Hard MIDI Reset within the pc program. At this point all the LED lights went out. It acted in a more normal manner. I.E. Moved adjust and the Edit led would light. But, disconnected the power, then re-applied the power and all LEDs came back On. So, hard reset again to get rid of their On status. Then adjusted and saved a few different effects to check it was a bit normal. Then unplugged unit. Turned On unit holding the Store button. When Store AND Tap are both flashing, pressed Store again and wait. This was the final process to get it back to factory settings. Don't know if you've still got you MPX, but I wanted to get the answer out there! Mines working great.