Recently I had an issue with logging in to my support agent account on our subdomain.

Error message "Forbidden - Invalid authenticity token"

No reset of password was possible at all.

Cause: Zendesk login requires 3rd party cookies to be allowed.

Solution: Either - allow third party cookies OR make 3rd party cookie exceptions as below.

Chrome - 3rd party cookie settings.

Chrome Settings
Search for Cookies
Click on "Cookies and other site data"
Scroll past the general settings and find "Customised behaviors"
Add sites to the 'Sites that can always use cookies' list, and include 3rd Party Cookies.

Refresh your page, and log in!