On Streambox Media Player for OSX, v 3.165.05 and previous versions, the Blackmagic Hardware Output function (requires activation) does not work with Blackmagic Drivers version 10.10, which was released on May 18, 2018. We are working to remedy this problem. The Blackmagic Hardware Output is the only affected feature, as far as we are aware.
In the meantime, downgrade your Blackmagic Video application to version 10.9.12, the next previous version, which can be found at: https://www.blackmagicdesign.com/support/family/capture-and-playback, under "Latest Downloads." Be sure to select version 10.9.12.