I hope this helps someone. I was able to fix this for my Hulu app on my Apple TV. The experience I was having was that I could use my password to log into the Hulu app on my iPhone and on the Hulu.com website. But when I tried to log in with that same password for the Hulu app on my Apple TV, it kept failing saying I had the wrong password. It’s weird because I was even using my iPhone to enter the password into the Hulu app on my my Apple TV, using the Paste function.
I recall having a similar issue months ago, when trying to log into the Disney+ app and it would log me out of Hulu. I’m not sure if you have the My Disney Experience (MDE) app (for visiting Disney World or Disneyland) but I do. So I tried to log onto the MDE app using the same password. And it failed. If you don’t have that app, maybe try to log into Disney.com. I was able to get into the MDE app to log me in via a one-time passcode since I didn’t know my password to MDE. Once in with the one-time passcode, I was able to go into my account on MDE and change the password. I changed it to the same password I still had in my iPhone clipboard, the password that worked consistently in the Hulu iPhone app and the Hulu.com website. Once that password change was done, I went back to the Hulu app on the Apple TV and once again used my iPhone to paste that same password into the password box in the Apple TV Hulu app and it logged me in.