I actually did not get that you do NOT need your Steemit-Password to log into DTube, but your Private Key.
Maybe some of you had the same problem, so here is the Solution (as described in this Post: https://steemit.com/dtube/@blondeism/how-to-join-and-use-dtube-from-my-experience-so-far-tutorial-on-how-to-set-up-an-account-and-log-in ):

1. Go to your "Wallet" on Steemit and click on the "Permissions" tab.
2. Under "Posting" there, click on "Show Private Key".
3. Copy your private key and log into Dtube with your Steemit username and this private key.