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" 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.