Here is the solution:
Credits to Morgonaut on Youtube
Uninstall Desktop Video App.
Boot to recovery mode and in Terminal execute this:
csrutil disable
Boot back to normal system and in Terminal execute this:
sudo rm -rf /Library/StagedExtensions/Library/Extensions/BlackmagicIO.kext
sudo rm -rf /Library/StagedExtensions/Library/Extensions/DeckLink_Driver.kext
Boot to recovery mode and in Terminal execute this:
csrutil enable
Install latest Desktop Video App and after reboot your Decklink or Intensity cards will be finally recognised by the Blackmagic Desktop Video app, enjoy
And if it's still not working then just paste this command to Terminal, hit Enter, type your password and hit Enter:
sudo kextutil /Library/Extensions/BlackmagicIO.kext
Best Chris