To stop automated attacks, sign-in, sign-up, password-reset and client sign-in-link forms use a security check from Cloudflare (Turnstile). Most people never see it: it runs in the background, and only shows a checkbox if something looks unusual.

• If it can't run (a content blocker, a very strict browser setting, a weak connection), the form says why and shows a Retry button.
• Turn off ad or privacy blockers for this site, or try another browser, if it keeps failing.
• Cloudflare is listed on our Sub-processors page for this purpose, and it receives only what it needs to tell people from automated programs.

Still stuck? See Can't sign in: the security check.