Error 520 Unknown Error — Causes & Fix Guide

Error 520 Unknown Error means the CDN received an unexpected or empty response from the origin server. The connection succeeds, but the reply is invalid.

This error is vague by design. It signals that something broke between your server and the proxy layer. The fix requires systematic checking.

Quick Fix

• Reload the page and clear browser cache.
• Check if your origin server is online.
• Disable problematic plugins or recent changes.
• Whitelist CDN IP ranges in your firewall.
• Restart Apache/Nginx and PHP.
• Check server logs for abnormal responses.

Most Error 520 Unknown Error cases are caused by server misconfiguration or security blocks.

What Is Error 520?

Error 520 is a generic CDN response. It appears when the proxy cannot interpret the server’s reply.

Unlike Error 521 or 522, this is not a timeout or a refused connection. The server responds — but the response is malformed, empty, or corrupted.

What typically happens:

• Visitor → CDN → Origin server
• Connection established
• Server returns unexpected headers or empty output
• CDN triggers Error 520

The root issue always lives on the origin server.

Why Error 520 Is Difficult

The message says “Unknown Error.” That means:

• No precise HTTP status returned
• No valid response body
• No proper header structure

You must investigate the server side carefully.

Main Causes of Error 520

1. Empty Server Response

The server closes the connection without sending data.

This often happens when:

• PHP crashes
• Memory limit is exceeded
• Fatal errors occur before output

2. Corrupted HTTP Headers

Improper headers confuse the CDN.

• Oversized headers
• Duplicate headers
• Invalid header formatting

Misconfigured security modules frequently cause this.

3. Firewall Blocking CDN Requests

Security systems may block proxy IP ranges.

When blocked, the server returns unexpected responses.

4. Broken WordPress Plugins

Faulty plugins can interrupt output buffering.

This leads to partial or malformed responses.

5. Resource Limits

If the server runs out of memory or CPU mid-request, it may terminate output.

Shared hosting environments are especially vulnerable.

6. Incorrect SSL or TLS Settings

Improper SSL configuration between CDN and server can trigger ambiguous errors.

Mismatched encryption modes are common.

How to Fix Error 520 (Step-by-Step)

Step 1 — Check Server Health

1. Access hosting control panel.
2. Review CPU and memory usage.
3. Restart web services.

If resources are maxed out, upgrade hosting or optimize workloads.

Step 2 — Review Error Logs

Open Apache or Nginx error logs.

Look for:

• PHP fatal errors
• Memory exhaustion
• Unexpected shutdown messages

Logs usually reveal the real trigger.

Step 3 — Disable Recently Added Plugins

If using WordPress:

1. Disable plugins one by one.
2. Test after each change.
3. Identify conflicts.

Security plugins are common culprits.

Step 4 — Whitelist CDN IP Ranges

Add official proxy IP ranges to your firewall allowlist.

Reload firewall rules and retest.

Step 5 — Increase PHP Memory Limit

Low memory causes abrupt termination.

Set:

• memory_limit = 256M or higher

Then restart PHP.

Step 6 — Check Header Size Limits

Oversized cookies or headers can exceed limits.

Reduce:

• Cookie size
• Custom headers
• Security module verbosity

Step 7 — Test Without CDN

Temporarily bypass CDN.

If the site works directly via IP, the issue is integration-related.

Advanced Troubleshooting

Analyze Network Traffic

Use diagnostic tools to inspect server responses.

Confirm proper HTTP status codes.

Review Security Modules

ModSecurity or similar systems may alter responses.

Temporarily disable for testing.

Check Keep-Alive Settings

Improper connection handling can create unstable responses.

Inspect Output Buffering

Ensure PHP output buffering is properly configured.

Prevention Tips

Use Reliable Hosting

Stable infrastructure prevents malformed responses.

Monitor Server Logs Daily

Early detection avoids prolonged outages.

Limit Heavy Plugins

Complex stacks increase risk of unexpected output.

Maintain Clean Header Structure

Avoid unnecessary or oversized headers.

When to Contact Support

Escalate to hosting provider if:

• Error persists after restart
• Logs show system-level faults
• Network routing appears unstable

Provide timestamps and log excerpts for faster diagnosis.

FAQ

Is Error 520 a server or CDN issue?

It originates on the server. The CDN only reports the invalid response.

Can WordPress cause Error 520?

Yes. Plugin conflicts and memory exhaustion frequently trigger it.

Does Error 520 affect SEO?

Repeated failures can reduce crawl efficiency and reliability signals.

Is Error 520 temporary?

Sometimes. But recurring instances indicate configuration problems.

How is Error 520 different from 521 or 522?

521 = connection refused. 522 = timeout. 520 = unexpected response.

Final Thoughts

Error 520 is vague but fixable. It almost always points to malformed output, server limits, or security misconfiguration.

Focus on logs, headers, and server stability. Clear diagnostics prevent repeated failures.