I just upgraded to X3. I only have one issue: No MIDI playback on any Kontakt instrument. I can click on the instrument interface keys and play a note. I have MC6 installed as well Kontakt instrument tracks play back without issue. My X3 is fully patched and I have updated Kontakt as well. Any suggestions appreciated. BTW, I initially installed the 64 bit version, uninstalled and installed the 32 bit version. The issue doesn't appear to be related to this. My version is KOMPLETE ELEMENTS.

When upgrading was the VST Inventory reset and rescanned? See http://www.cakewalk.com/Support/Knowledge-Base/2007013315/SONAR-X3-Full-VST-Rescan-Recommended-for-Upgraders
 
Are these existing projects made in MC6 or new X3 projects?

Yes the VST dir was rescanned. The problem is with a new project and existing projects. When I open piano roll for a track with a Kontakt instrument, I can play a note on the keys. I did perform the VST Inventory reset as you requested with no result.

Do Cakewalk instruments work?

I found one way around the issue. In the Kontakt instrument interface, the instrument MIDI port defaults to Port A [1]. Setting it to Omni works.

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Irrelevant comment deleted.