Are you able to login to Hulu on all your other devices except Apple TV? I have a fix which will allow you to login on Apple TV too.
I pride myself in knowing all my account passwords but last weekend something happened to my Hulu account on Apple TV where I could not log in. I was able to log in using my account on Hulu.com, same with my Hulu app on my iPhone, but when using the same user name and password on the Apple TV it would say that the user name and password combination was not valid. After literally validating it no less than 10 times by trying to login on every other device successfully, I finally figured it out.
I also a separate Disney+ account. I tried logging in with that and it worked. If you are experiencing the same thing – try your Disney+ account and I bet that will work.
I wish I would have thought about this earlier – but hopefully you did a search and found this to help you keep your sanity when trying to login to Hulu on your Apple TV.