I already reported this here a few months ago: https://forums.boxcryptor.com/topic/boxcryptor-classic-on-osx-1010-yosemite. But my message has remained without any answer. So I took it upon myself to post this again in a new topic.
Boxcryptor classic is working fine on OS X 10.10(.2), the only small glitch (but rather annoying) being visual: when using the "Dark Mode" on OS X, the boxcryptor status/tray icon stays black and is barely visible (instead of being "inverted" to white).
I don't know if Apple will add something to their UI to make the inverting possible, or if every app developper is supposed to add a white/"dark mode" version of their icons.