I've not been able to update Cyerduck in weeks. I opened an older ticket about this but don't remember the name. since that ticket I'm experiencing the same problem. I open cyberduck and it prompts me to update. I do. It downloads the app. It installs the app with the "Updating Cyberduck" window. It gets stuck at this point. See the screenshots.
I should also note that I've updated to os x 10.10.5.
I'm on a MacBook Pro Retina, 15 inch, mid 2015)
I restarted my computer to see if this had an impact on updating Cyberduck but no change. The same behavior happens.