Error Code 520 : Causes, Fixes, and Troubleshooting Guide
Henrik Olaf Nilsson
2026-08-04 16:00
You open your website monitoring dashboard and suddenly see an Error code 520 alert. The server is still running, and there are no obvious errors in the logs, but the website is unavailable. This situation is often confusing because everything appears normal on the surface.So what exactly causes this problem?
An error code 520 usually means that the origin server returned a response that Cloudflare could not properly understand. The problem is not whether the server can be connected, but what the server actually returned.
Unlike other Cloudflare-related errors, such as 521 or 522, the focus of a 520 error is the content of the response from the origin server rather than whether the server responded at all.This issue may be caused by application problems, server configuration errors, request processing failures, or abnormal responses from backend services. It can also appear in automated data collection scenarios where requests rely on stable server responses.
This article will explain what Cloudflare error code 520 means, why it happens, and how to troubleshoot and fix it.
Error code 520 is not part of the official HTTP status code standard. Instead, it is a custom error message used by Cloudflare to indicate that the origin server returned an unexpected response.
In simple terms, a web server is returning an unknown error error code 520 means that the server did respond, but the response was not something Cloudflare could correctly process.Cloudflare works as an intermediary between users and origin servers. When a user visits a website, the request first goes through Cloudflare, which then communicates with the origin server to retrieve the requested content.
If the origin server returns an empty response, an invalid format, or incomplete data, Cloudflare may display a 520 error code.Simply put, a 520 status code means:The origin server returned a response, but Cloudflare could not understand it.
For example, imagine asking someone to pick up a document for you. They arrive at the office, but instead of receiving a valid file, they get a blank page or nothing at all. Since they cannot confirm the information, they return and tell you that something went wrong. This is similar to why a website may display a 520 error message.
One of the most common causes of error code 520 is that the origin server sends back a response that Cloudflare cannot interpret correctly.When Cloudflare forwards a request to the origin server, the server needs to return valid response data. However, problems may occur if the server returns empty content, an abnormal response format, or closes the connection before completing the request.In this situation, the server itself may still be online and running normally. The issue is that the returned information does not meet the requirements needed for proper processing.
Modern websites usually depend on multiple components, including website applications, databases, APIs, and backend services.If any part of this system fails, it may affect how the server responds to user requests.For example:
These problems can cause the server to generate unexpected responses, which may eventually trigger a Cloudflare error code 520.
Sometimes, the problem is related to how the server handles incoming requests.For example, abnormal request headers, oversized cookies, incorrect parameters, or unsupported request formats may prevent the server from processing the request properly.Server-side rules can also contribute to the problem. Incorrect firewall settings, security policies, or access control configurations may interfere with normal request handling and cause the server to return an unexpected response.
When website traffic increases, servers may experience resource limitations or unstable backend services.If the application cannot complete request processing correctly, the server may return incomplete or abnormal data, resulting in an http error 520.Unlike timeout-related errors, a 520 status code is usually not caused by the server taking too long to respond. Instead, the issue is that the response itself contains something unexpected or invalid.
Cloudflare provides several 5xx-level error messages, and they are often confused because they all indicate problems between the user and the origin server.
However, the causes and troubleshooting methods are different.
| Error Code | Meaning | Main Problem Location |
| 520 | Origin server returned an empty or unrecognized response | Website application or origin server response |
| 521 | Origin server refused the connection | Server service, firewall, or connection settings |
| 522 | Connection timeout between Cloudflare and origin server | Network connection or origin server availability |
| 524 | Connection established, but origin server took too long to respond | Slow application process or long-running requests |
The troubleshooting direction depends on the type of error.For a 521 error, the first step is usually checking whether the server service is running or whether a firewall is blocking requests.For a 522 error, the focus is on whether Cloudflare can successfully connect to the origin server.For a 524 error, the problem is usually related to slow processing, database queries, or long execution times.
However, when dealing with error code 520, the key question is:What exactly did the origin server return that Cloudflare could not process?
Although error code 520 is not specifically designed for data collection, similar issues may appear during automated requests, API calls, and other data access processes.
Automated data collection tasks often need to access multiple pages or APIs.If the target website has unstable server responses, or if the request processing process returns unexpected content, Cloudflare may receive a response that it cannot properly interpret.In this situation, 520 error code does not necessarily mean there is a problem with the data collection script itself. The issue may come from the target server, backend application, or the way requests are handled.
Some data collection processes depend on APIs provided by websites.If an API returns incorrect data formats, experiences service failures, or cannot complete backend processing, the requesting system may not receive the expected results.At the same time, the server may return abnormal responses, which can lead to a 520 status code.When troubleshooting this type of issue, it is important to check both the request logic and the API response to determine where the problem occurs.
Automated requests are different from normal browser visits.Changes in network conditions, request parameters, or access environments may affect how the target server processes requests.If the server application cannot properly handle these changes, it may return an unexpected response and trigger a web server is returning an unknown error error code 520 message.In practical operations, a stable request environment can improve the reliability of automated tasks. If response failures occur frequently, checking request conditions and access stability can also be part of the troubleshooting process.
The key to fixing error code 520 is identifying why the origin server returned an unexpected response.The troubleshooting process can usually start from three areas: server status, application performance, and request handling
The first step is to confirm whether the issue comes from the origin server itself.You can directly test the origin server response and check whether it can return content normally.For example:
curl -v -H "Host: your-domain.com" http://your-origin-ip/If the origin server also returns an abnormal response, the problem is likely related to the server application, configuration, or backend services.If the origin server works normally but the website still shows Cloudflare error code 520, further investigation should focus on CDN settings, request processing logic, and the response returned by the server.
Server logs are one of the most important sources of information when troubleshooting a web server is returning an unknown error error code 520 issue.Check the time when the error occurred, related requests, and application error records.You should also review:
If the server has insufficient resources, unstable services, or incorrect configurations, correcting these issues can help restore normal responses.
Website applications, databases, plugins, and APIs all affect how requests are processed.During troubleshooting, check whether:
If the application cannot process requests correctly or an API returns unexpected data, Cloudflare may receive an invalid response and display error code 520.
If the issue appears during API calls or automated requests, review the request itself.Check whether:
Improving error handling and optimizing request logic can help identify the source of the problem faster and reduce repeated response failures.
Q: What does error code 520 mean?
A: Error code 520 means that the origin server returned a response that Cloudflare could not properly understand. It does not always mean that the server is offline. In many cases, the issue is related to the returned response content.
Q: Is error code 520 always caused by Cloudflare?
A: No. Cloudflare only displays the error result. The actual cause is usually related to the origin server, website application, server configuration, API response, or request processing.
Q: Why does error code 520 disappear after refreshing the page?
A: This usually happens because of temporary server response issues, resource pressure, or short-term application failures. If the problem happens repeatedly, server logs and application status should be checked.
Q: Can restarting the server fix error code 520?
A: Restarting a service may solve some temporary problems, but it cannot fix every 520 issue. If the cause is related to application logic, configuration errors, or request handling, further troubleshooting is still required.
Q: How long does error code 520 last?
A: The recovery time depends on the actual cause. Temporary service issues may recover automatically, while problems related to applications, configurations, or backend services require manual fixes.
Error code 520 is a signal that the response returned by the origin server could not be properly processed.It does not necessarily mean that the website is completely down. In many cases, the cause may be a configuration issue, application error, or temporary server instability.
The best way to solve a 520 error code problem is not simply refreshing the page or restarting the server, but following a clear troubleshooting process — checking logs, reviewing configurations, and analyzing how requests are handled.Once the source of the abnormal response is identified, most 520 issues can be resolved effectively.