Ok. I sended a small amount to bittrex using ledger and it worked like that.

When I tried to send a transaction as usual there appeard a messagebox named "Ledger Wallet Authentication".
Looked like this

It asked me to match the character between the stars with my security card.
In this case the *2* was meant. So I took my security card got to "Digits" and looked what character or number was there below (white sign on black background). I typed this in the blank field and clicked ok.
Then it asked me again but a different character was put in stars. I looked again at my security card for the character. Typed in the character/number below it and pressed ok.
I had to do this process 4 times and finally the payment was send.
That's it