In order to resolve this problem, please follow these steps:
- Go to System Preferences -> Security & Privacy -> Privacy tab -> Full Disk Access
- Enable changes by clicking the pad lock in the lower left.
- Drag "MacUpdate Desktop.app" from your applications folder to list of Allowed apps.
- Try relaunching MacUpdate.
Disclaimer: make sure to select "Full Disk Access" in the list on the left, not "Accessibility" that is selected by default.