While starting up, Unicorn will get stuck in a loop of
INFO -- : Refreshing Gem list
INFO -- : Refreshing Gem list
INFO -- : Refreshing Gem list
No error message will show up.
I figured out the problem and want to post the answer here, should someone else run into this problem.
Make sure your code is free of syntax errors!
For me fixing a syntax error (in one of my controllers) ended the loop and started Unicorn properly. I did not get any error messages in Unicorn, you might want to try to start using WebRat and see if an error pops up.