Which version of MadMapper ?
Which version of the BlackMagic Desktop Video ? (it must be at least the version we used for building - in this MM 2.5.0 release it need BlackMagic Desktop Video 10.6 or later)
Another common problem is that MM opens the default audio input of the computer. If a blackmagic source is selected as default audio input, it will open it, and then it's impossible to open the video stream (we're not responsible for that...) However I think in latest release we refuse to use BlackMagic audio input for audio analysis to avoid breaking video input.
And finally, be careful to stop any other application that opens BlackMagic driver because it accepts only one software to use it at a time.
Let us know
Matthieu - MM team