ERR_TOO_MANY_REDIRECTS: redirected too many times

daniele@mikweb: ~/errors — less err-too-many-redirects-redirect-loop.md

What the message looks like

This page isn't working
example.com redirected you too many times.
ERR_TOO_MANY_REDIRECTS

A CDN or proxy talks plain HTTP to your server, while the server redirects every HTTP request to HTTPS (a typical Flexible SSL loop).
The application builds the redirect from the wrong scheme because the proxy does not pass X-Forwarded-Proto, so it redirects to HTTPS again and again.
Conflicting www and non-www rules: the server redirects to one, the application or CMS to the other.
A CMS has siteurl or home set to a different address than the one visitors use.
Stale cookies in a login redirect loop (works in a private window).

Follow the redirects with the limit and read each Location
curl -sIL --max-redirs 10 https://example.com/ | grep -iE '^(HTTP|location)'
What does plain HTTP do?
curl -sI http://example.com/ | grep -iE '^(HTTP|location)'
Redirect rules in the nginx config
nginx -T 2>/dev/null | grep -nE 'return 30[0-9]|rewrite .*(permanent|redirect)|X-Forwarded-Proto'
Test with the proxy header the app expects
curl -sI -H 'X-Forwarded-Proto: https' http://127.0.0.1/ -H 'Host: example.com' | grep -iE '^(HTTP|location)'

Read the Location chain from the first check: the two URLs that alternate show who redirects where.
With a CDN, set its SSL mode so it connects to your origin over HTTPS (for example Full), not plain HTTP.
Behind a proxy pass the scheme: proxy_set_header X-Forwarded-Proto $scheme;, and make the application trust it.
Decide on one canonical host (www or not) and redirect only once, in one place.
Fix siteurl and home of the CMS, clear the site cookies, and test again.

A cookie, often a session or login cookie, takes part in the loop. Clearing cookies for the site confirms it.

A limit exists and differs per browser, so do not rely on a number. Any loop eventually ends in this error.

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