Mike, The symptoms you describe suggest that electrical noise is
corrupting the MIDI signals, most likely generated in the PianoDisc
unit and conducted via the power lines. The UPS unit isolates the PC,
just as the battery-operated laptop is isolated.
A possible cause of the problem is that either the PC's MIDI adapter &
cable or the PianoDisc, or _both_, don't conform to the MIDI Association
electrical design specification, which requires an opto-isolator circuit
on all MIDI ports.
The specs for both devices should say that they are optically isolated
in conformance with the MIDI hardware specification. If they don't
conform, then complain!