can not change iCloud password

I use iMac early 2009

Browser: Chrome and safari

I tried to change my iCloud password several times.

Started from reset my current password to login with security question and change my current password.

But after I changed my password, it brought me to the login page with a message "An unknown error has occurred."

And now I still sign in with my old password.

Anybody help me solve this problem, please.

Thank you.