Using Openstudio 2.3.0 with Build Number: cf58ee1e38 on a Macbook Pro 2017 running macOS Sierra 10.12.6
Can be recreated by opening the OS app, navigating to Components & Measures > Find Measures, selecting a measure and pressing download. The BCL pop up remains working and more measures can be downloaded, however after closing the BCL pop up all OpenStudio buttons become inactive.
Going to Apple > Force Quit doesn't show OpenStudio as 'Not Responding'.