For possibly various reasons, with recent updates a number of third party apps can’t properly access virtual webcams such as ManyCam or CamTwist on macOS. The two that affect me are Zoom and Camtasia. The temporary workaround is to remove the code sign signature from the target app’s binary. In the case of Camtasia 2020 you’ll need to run this command in a terminal:
sudo codesign --remove-signature /Applications/Camtasia\ 2020.app/Contents/Resources/CLRecorder
This relies on having Xcode installed, so if you don’t have it, you’ll need to run this first:
Naturally, removing a code signature is not ideal, but hopefully the various parties involved will be able to figure out a way forward. It is also possibly better than reverting to an older version of the software, since particularly in Zoom’s case they’re focusing on security updates.
Hope this helps anyone searching for this issue.