Marked as helpful by the asker
Cloudflare SSL/TLS mode is on Flexible. That means Cloudflare talks to Vercel over plain HTTP. Vercel redirects HTTP to HTTPS. Cloudflare gets the redirect, passes it to the browser, browser asks Cloudflare over HTTPS again, Cloudflare asks Vercel over HTTP again... loop.
Cloudflare dashboard → SSL/TLS → set mode to Full (strict). Loop gone within a minute.
Even simpler: set the record to DNS only (grey cloud). Vercel already does CDN and certificates, running Cloudflare's proxy in front of it mostly adds a second layer to debug. That also clears the warning on Vercel's domain page.
Full (strict), fixed instantly. Going grey cloud too, I don't need two CDNs emeka
Happy to help. Keep an eye on the domain page in Vercel after going grey, the warning should clear within a few minutes. jonas_k
jonas_k This is the #1 Cloudflare question I see anywhere. Should be pinned somewhere. cloudkat
Same fix for Netlify behind Cloudflare. 
thandeka
thandeka 