Captcha enabled but not configured
Captcha is turned on for a surface (often comments on the download page), but the selected provider has no usable public/secret keys — so the UI cannot show a working challenge.
Why this happens
captcha_mode is set and a feature toggle (e.g. comment captcha) is on, while reCAPTCHA / Turnstile / hCaptcha keys are empty or incomplete. Download-grant flows may skip or warn differently; comment UI often blocks with this exact line.
Diagnose and fix
1. Admin → Settings → Main → Captcha — pick provider (captcha_mode) and fill keys (reCAPTCHA v2/v3, Turnstile, hCaptcha).
2. Enable only the surfaces you need (Captcha on comments, login, registration, tickets, etc.).
3. For download-page captcha per tier: Users & limits → Download captcha (toggle.ul2_cap_dl_*).
4. Save and hard-refresh the download page; test with a private window.