C1 is played at C3 in Modplug.
That's because the lowest octave in OpenMPT is octave 0, the lowest MIDI octave is -2 (yep, that's a minus in front of that 2). So if you trigger a "C-0" (C zero) in the pattern, it is really a C-2 (C minus two) in MIDI.
Anyway, OpenMPT has a note range of 10 octaves (C-0 through B-9), that's exactly 120 notes, but there are MIDI notes, so the upper 8 notes are not accesible through pattern notes, so this limitation is "by design". However, there are still ways to access those 8 notes, as described here
, so I'm going to move this old report.