Error type: Cloudflare Origin Error
Error code: 520
Also appears as: “Error 520: Web Server Returns an Unknown Error”

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A 520 error means that Cloudflare successfully connected to your web server, but the response it received back was empty, invalid, or didn’t follow standard HTTP protocols. Rather than pass a broken response on to your visitor, Cloudflare returns a 520 error page instead.

In short: your server is responding, but not in a way that Cloudflare can interpret or pass on.

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Not necessarily. Your web server may still be running, but something is causing it to return an unusable response. The distinction matters because it changes where you need to look to fix the problem.

For reference, here’s how 520 relates to similar Cloudflare errors:

• 520 — Server responded, but with something invalid or empty
• 521 — Server actively refused the connection
• 522 — Connection timed out before a response was received
• 523 — Cloudflare cannot reach the origin server at all

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There are several common causes. The most frequently seen are:

The server sent back a completely blank reply — no headers, no content. This often happens when a PHP script crashes mid-execution before sending any output to the browser.

A broken plugin, a failed update, or a theme conflict can cause WordPress (or another PHP application) to throw a fatal error. On a live site with error display turned off, the server may return nothing useful — triggering the 520.

HTTP response headers must follow a specific format. If your server sends headers that are malformed or unusually large, Cloudflare will reject them and return a 520.

Running out of PHP memory, database connections, or processing capacity can cause the server to fail mid-response. This is particularly common under sudden traffic spikes or on resource-constrained hosting plans.

On Apache-based servers, a misconfigured .htaccess file can cause the server to behave unexpectedly. Even a single erroneous character can prevent requests from being processed correctly.

A server-side firewall or security tool may be blocking requests from Cloudflare’s IP ranges. If the server doesn’t recognise Cloudflare as a trusted source, it may respond abnormally or not at all.

A mismatch between the SSL mode set in Cloudflare and the actual SSL certificate configuration on your origin server can result in an invalid or refused response.

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Your hosting control panel (cPanel, Plesk, or similar) will give you access to your server’s PHP and Apache/Nginx error logs. These often identify the exact cause — a memory limit exceeded, a fatal PHP error, or a specific file causing problems.

Try accessing your website using its origin server IP address rather than the domain name. This bypasses Cloudflare entirely. If the site loads correctly this way, the issue lies in how your server is responding to Cloudflare specifically, rather than a general outage.

Log in to your Cloudflare account and check the Analytics and Error Analytics sections. Cloudflare will often provide additional context about the type of invalid response it received and when errors began occurring.

Consider what changed around the time the errors started. A plugin update, a new deployment, a scheduled task, or a configuration change are all common triggers. Reversing or reviewing recent changes is one of the quickest ways to narrow down the cause.

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If a PHP fatal error is causing an empty response, enable WP_DEBUG in your wp-config.php on a staging or development environment to surface the error message. On a live site, review your error logs instead. Deactivating recently updated or added plugins one by one is a reliable way to identify a problem plugin.

If PHP memory limits are being exceeded, increase the memory allocation in your php.ini file or by adding define('WP_MEMORY_LIMIT', '256M'); to your wp-config.php. Your hosting provider can assist if you’re unsure of the appropriate limit for your plan.

If you suspect a corrupted .htaccess file, replace it with a clean default version. In WordPress, you can regenerate a fresh .htaccess file by navigating to Settings > Permalinks in your dashboard and clicking Save Changes — no other changes are required.

If your server-side firewall is blocking Cloudflare’s requests, add Cloudflare’s IP ranges to your server’s allowlist. Cloudflare publishes its full, up-to-date list of IP addresses at cloudflare.com/ips. Your hosting provider or server administrator can add these to your firewall rules.

In your Cloudflare dashboard, navigate to SSL/TLS > Overview and check which mode is selected. Ensure it matches your server’s actual certificate setup:

• Flexible — No SSL certificate on your origin server
• Full — SSL certificate present but may be self-signed
• Full (Strict) — Valid, trusted SSL certificate installed on the origin server

A mismatch here — for example, Full (Strict) selected but no valid certificate on the server — is a common and easily overlooked cause of 520 errors.

As a diagnostic step, you can enable Development Mode in the Cloudflare dashboard under Caching > Configuration. This temporarily bypasses Cloudflare’s cache and some processing. If errors cease in Development Mode, it helps confirm that the issue is related to how Cloudflare is interacting with your server, rather than the server itself being broken.

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Intermittent 520 errors are often a sign of a server under resource pressure — the error occurs during periods of high load or when certain resource-intensive processes (such as a scheduled backup or a WooCommerce import) are running. In this case, reviewing your hosting plan’s resource limits, optimising database queries, or upgrading your hosting environment may be necessary for a permanent resolution.

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If you’ve worked through the steps above without success, or if you don’t have access to server error logs and configuration files, then yes — this is the point to bring in professional support. A 520 error rooted in server configuration, firewall rules, or application-level PHP issues will require access to the server environment to resolve properly.

If your website is hosted or supported by O’Brien Media, please get in touch with our support team and we’ll investigate on your behalf. You can reach us on 01793 239239, by email at [email protected], or via the Help & Support section of your Client Connect portal.

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Possible Cause	Where to Check	Fix
PHP fatal error	Server error logs / WP_DEBUG	Identify and resolve the error; deactivate problem plugins
Empty server response	Server error logs	Investigate crashed scripts or processes
Memory limit exceeded	Server error logs / php.ini	Increase PHP memory limit
Corrupted .htaccess	File Manager / FTP	Replace with clean default; regenerate via Permalinks
Cloudflare IPs blocked	Server firewall / hosting panel	Whitelist Cloudflare IP ranges from cloudflare.com/ips
SSL/TLS mismatch	Cloudflare dashboard > SSL/TLS	Align Cloudflare SSL mode with origin server certificate
Server resource exhaustion	Hosting control panel / logs	Optimise resource usage or upgrade hosting plan