MiDi signal received but no sound in MainStage patches

While using MainStage, my patches suddenly stop working: I can see that the MiDi signal is received (keys go down in mainstage), but no sound is forwarded. This might affect 25 out of 30 of my patches.

What could cause this? I have tried to disable autosave and free up some CPU, but I would like to know for sure what is causing the issue instead of hoping that it won't happen again.

I cannot reproduce the issue at will.

[Re-Titled by Moderator]

Original Title: Patches suddenly stops working