We've had some reports of Ecamm Live freezing up on launch after updating to macOS 10.15 (Catalina). Fortunately, it's an easy fix.
If you see this happening, it may be caused by the EpocCam plugin from Kinoni. It appears to be causing the Mac to freeze whenever it is used.
Since Ecamm Live attempts to start all camera devices whenever it starts, it runs into the problem on launch.
To fix this, download and run the EpocCam installer. This will update EpocCam to the latest version, which has fixed the issue.
Once you've updated EpocCam, if EpocCam is in the Camera menu but not in the camera switcher, pick EpocCam in the Include in Switcher menu in the Camera menu at the top of the screen in Ecamm Live to re-add it to the switcher.