Time Machine Backups Could Not Be Disabled
macOS 10.15 (Catalina) and later require that applications be granted the "Full Disk Access" privilege to enable or disable automatic backups. If you use the "Disable Time Machine backups while connected" feature without granting Viscosity this privilege, then you may see the alert below.
To resolve the issue, click the "Open System Preferences" button to be automatically taken to the necessary section in System Preferences. You can also get to this section manually by going to the Apple menu, selecting "System Preferences", clicking on the "Security & Privacy" icon, and clicking on the "Privacy" tab.
Then click the small padlock icon in the bottom left of the window and enter your macOS username and password so you can make changes to the settings.
Once unlocked, scroll down the list on the left until you see "Full Disk Access", and select it. Then scroll the list on the right until you see "Viscosity.app" (Note: On macOS 10.15 the entry will instead be named "com.sparklabs.ViscosityHelper"). Tick this item, close the window, and you're done.
If you don't see "Viscosity.app" listed, you may need to restart your computer, and then connect a VPN connection again so Viscosity can attempt to disable Time Machine again.