1. Make sure C:\Program Files\Quest Software\Toad for Oracle 2020 R2 Edition\Toad for Oracle 14.2\SQLTracker\sqldbmon.exe is on your firewall/anti-virus safe list.
2. On the sqldbmon.EXE, right click | Properties | Compatibility and select "Run this program on compatibility mode" and test with the other versions. Then try running SQL Monitor.
3. On the sqldbmon.EXE, right click and choose "run as administrator". Please test this even though you are already an admin user.
See another workaround for a specific scenario
here.