Q: cannot sign in to iCloud account

When I log into the iCloud web interface, after typing in my Apple ID and password, the next window asks me two security questions, which I answer correctly, but instead of being in the iCloud web interface now, I am asked to type in my password again and then choose a new password and confirm that. I've changed my password twice now, and it continues this loop again and again.

Each time I try to log in with the new password, it works on all of my other devices, but not my Macbook pro. There I get the message, " an unknown error occurred."

I don't want to change my Apple ID password again! I used to just be able to log in without getting all these questions.

This is very frustrating as my devices are no longer synced.

All of my devices are up-to-date, with my Mac Pro running OS X 10.11.6. Can anyone help me?

Thanks.