[take77]

Something similar happened using one of the a bundled PT instruments. I opened a second instance of Mini-Grand on an adjacent instrument track. I could see the MIDI meter was active but no audio signal coming through the instrument fader. I double checked the MIDI output routing and made sure it was output to "MG2" but no audio signal. I could however, route it to the MG1 track and get a signal.

So I created a new instrument track fresh for Mini-Grand 2, assigned the output accordingly & then dragged the midi file from the previous track onto the new track and it worked fine.

Since the session was was older, so I don't know if I had duplicated the previous track and it's properties or if that might have had something to do with it.
In any event, will let you know if it happens again.

Are you using SSD 4? I use SSD 3.5 within Kontakt 5.5 nearly every session and haven't experienced any problems.