The same Problem on OSX 10.9.5 !!!
Have you tried logging out of your CC desktop app, then logging back in again? Quit/Restart won't work, but logging out and back in should.
Signing out did the trick - thanks! Not very obvious, but it looks like it needed to refresh the license from Adobe.
When I started Lightroom after signing out and back in to the CC application, Lightroom prompted me to confirm the account that I was using. It showed a wizard that at one point said my trial had zero (00) days remaining. After clicking through the wizard and at some point clicking a "Sign In" button, it worked a charm.
Thank you too... I downloaded this morning, and opening up, only briefly showed the splash screen and then nothing... logging out of my CC and logging back in did the trick and now have LR6! So, thanks for helping me too!
All the best to you.
Thanks - that's worked for me too. I would have never have found that!