I have the same problem since it started. Every once in a while I try it but nothing. I think there might be quite a few posts out there about this.
https://steemit.com/steemit/@musicbox/how-to-dtube-having-problems-login-in-dont-know-how-to-use-it

https://steemit.com/dtube/@bumsbud/dtube-error-private-posting-key-is-wrong

https://steemit.com/dtube/@thatdude/ahha-figured-out-dtube-login-thanks-to-ajvest

...upvoted and resteemed