What it means
Cloudflare received an empty, invalid or unexpected response from your origin server. Server errors mean a valid request failed on the server or a service behind it.
Common causes
Origin crashed, oversized headers, or the origin closing connections early.
How to fix it
Check origin logs, restart the app, and make sure headers stay under limits.
If you are a visitor
The problem is on the website side. Wait a few minutes and reload. If it persists, check whether the site is down for everyone with the site down checker.
If you own the site
Retrying later often works. If it keeps happening, the site owner must fix the server. Short 5xx outages are usually tolerated, but errors lasting hours can make Google slow its crawling, and days of errors can drop pages from the index. A 503 with a Retry-After header is the right signal for planned downtime. Run the HTTP status checker to see exactly what your URL returns, and use uptime monitoring to be told when it changes.
Frequently asked questions
What does HTTP 520 mean?
Web Server Returned an Unknown Error (Cloudflare): Cloudflare received an empty, invalid or unexpected response from your origin server.
Is HTTP 520 an error?
Yes. Server errors mean a valid request failed on the server or a service behind it.
Does HTTP 520 affect SEO?
Short 5xx outages are usually tolerated, but errors lasting hours can make Google slow its crawling, and days of errors can drop pages from the index. A 503 with a Retry-After header is the right signal for planned downtime.
Should I retry after HTTP 520?
Retrying later often works. If it keeps happening, the site owner must fix the server.