Pretty much what it says on the tin. I quit out of the game, and the next time I started up Island Rum, it said it was downloading a new version of itself. It did that and then restarted itself. The restarted Island Rum renders no windows, it just runs/hangs and does near as I can tell nothing at all. I downloaded the latest version and replaced it, and rebooted my machine. Newly downloaded Island Rum behaves identically: hang, unresponsive, nothing happening.
I'm on an iMac (Retina 5K, 27-inch, 2019), macOS Catalina.