Alrighty then, now you're looking for a connection issue in the strip, or potentially an issue with the 03 Multiplex card. Measure the voltages from the fader to the 13 VCA card edge connector: 0v on pin 33, 5v on pin 35, and 0-5v on pin 25, the fader wiper. If they're good, look for 0-5v on pin 17, Computer Send. If that's good, look for the same 0-5v on pin 20, Computer Return. If that's bad, swap the Multiplex card and check the S21 ribbon is properly seated; that's a very common fault.
Chris