If you receive an error message: "An Audio Unit plug-in reported a problem which might cause the system to become unstable. Please quit and restart MainStage" the most common reason for this error is a third party plugin you've recently installed that isn't working correctly within MainStage.
You can view all plugins installed on your computer by going to MainStage menus > Preferences > Plug-In Manager.
From there you might check on any compatibility issues or delete any that you don't need anymore.
Click here to read more for additional help with third-party Audio Units Plug-ins.