A login that returns you to the login page instead of the account usually has a simple cause.
Reload cleanly
Open a fresh copy of the login from a verified address rather than a page that has been sitting open. A stale page can carry an expired session or captcha.
Allow the site its cookies
Tor Browser at Safest still allows first-party cookies, which the session needs. If you have manually blocked cookies for the site, the login cannot hold a session and will loop.
Type into the real field
The decoy login fields trap automated fillers. If your password manager is filling a honeypot, the real field stays empty and the login fails. Fill the visible field by hand.
Confirm the address
A clone can loop you deliberately while it captures what you type. Confirm the captcha address matches your bar before trying again.