Have you verified that the account you are logging in with has the appropriate permission, on the appropriate project, to publish content?
That's the only thing I can think of off the top of my head that might cause a response like this.
You might also try this:
Before you go to publish anything, simply go into Tableau Online and select the "Server" menu option. Then select "Sign in". Is it currently pointing to anything, such as public.tableau.com? If so, choose 'sign out' and try again with 'https://online.tableau.com/".
Best of luck! Let us know if you find a resolution and what is was.
Tableau Online Deployment Advisor