Random instrument channels in existing projects suddenly inaudible

Updated to 12.0.70 recently. I’m not saying for sure it’s related to the problem I’m experiencing, but just for context …

I opened up a few projects I haven’t touched for a few months (and were working fine as memory serves) and I cannot figure out for the life of me why some of my instrument channels are not outputting audio. Nothing shows up on their respective instrument channel mix level monitors, but MIDI monitor on the “non-working” instruments show note activity during playback. The VST UIs (Massive, Serato Sample, XO) are showing activity as they receive midi input, just as they do on the “working” audible tracks. I just don’t hear anything on those “broken” tracks. The channels are set to the exact same outputs, channel groups, etc as other tracks that are audible. No amount of toggling of mute/soloing makes them audible. The project feels, for lack of a better word “broken”.

I can’t find any pattern in which instrument channels are producing audio and which are not in either project. In both projects, some of the Serato Sample, Massive and XO tracks work, others don’t. If I duplicate an instrument track that’s not outputting audio, the duplicated track doesn’t either. But I can create a new instrument track, select the same vst as a non-working track, and use the same vst settings from the non-working track’s vst, and that track outputs as I expect. About the only constant is all audio tracks in both projects are all working properly.

I’m really at a loss for how to drill down further. Clearly the instruments are getting midi output and they think they’re outputting audio, but nothing hits their respective channel mixers. It feels like the connection between the instrument vsts and the instrument track’s “audio input” is randomly broken on some tracks … but of course there is no input per-se because they’re instrument tracks. I’m not new to Cubase by a long shot, but I’ve hit a wall in terms of trying to debug and or repair these tracks. The problem on the broken tracks seems to be occurring before any audio even hits the instrument channel mixers. Hopefully I’m explaining this accurately.

Does anyone have any ideas on what to look at or try?

Are there by any chance additional FX plugins on those instrument tracks? Some plugins simply mute their own output if they think they are not licensed correctly, that is a common cause for the problem you describe.
You can test it by disabling (not just bypassing, you need to “ALT+click”) the plugins one by one.

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Nailed it in one, fese! My bx_subsynth plugin decided it was unlicensed (thus needed a quick re-login to plugin alliance)! Thanks so much, all my testing was just bypassing the inserts, but indeed de-activating them quickly helped me find the culprit.