Error map
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HTTP · layer 2, Network
Centre: this error. First ring: errors it is linked with (12). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (12)
CORS Error: No 'Access-Control-Allow-Origin' header is present
The browser blocked a cross-origin request because the server’s response didn’t include the CORS headers allowing your site’s ori…
Warning: session_start(): Session cannot be started after headers have already been sent
session_start() ran after output was sent, so PHP can’t set the session cookie.
Upload error: File too large (LIMIT_FILE_SIZE / file exceeds maximum upload size)
The uploaded file is larger than the limit set by the API, framework or server.
Cloudflare 413 Request Entity Too Large (upload over the plan limit)
The request body is larger than Cloudflare allows through the proxy — 100 MB on Free/Pro, 200 MB Business, 500 MB Enterprise defa…
WordPress: Sorry, you are not allowed to upload this file type / Sorry, this file type is not permitted for security reasons
WordPress blocked the upload because the file extension/MIME type isn’t in its allowed list (SVG, JSON, WEBP on old versions, fon…
HTTP 411 Length Required
The server refuses a request with a body but no Content-Length header.
Unexpected characters at the top of the page / headers already sent by BOM
A UTF-8 byte-order mark (BOM) at the start of a PHP file is sent as output, breaking headers, JSON and images.
Upload error: File type not allowed / unsupported file type
The upload was rejected because the file’s type — extension, MIME type or actual content — isn’t on the allowed list.
Cloudflare Error 1040 / 1041: Invalid request header modification
A Transform Rule tried to set or remove a request header that Cloudflare doesn’t allow to be modified, or produced an invalid val…
Warning: Cannot modify header information - headers already sent
You called `header()`, `setcookie()` or `session_start()` after output had already been sent to the browser.
HTTP 505 HTTP Version Not Supported
The server doesn’t support the HTTP protocol version used in the request.
Refused to load the script because it violates the following Content Security Policy directive
The page’s Content-Security-Policy doesn’t allow that script, style, image or connection source.
Two steps away (36)
AWS S3: AccessDenied (403) when calling GetObject / PutObject
An IAM policy, bucket policy, ACL/ownership setting, Block Public Access or KMS key policy denied the S3 request.
HTTP 401 Unauthorized
The request has no valid authentication credentials — you need to log in or send a valid token.
HTTP 403 Forbidden
The server understood the request but refuses to authorize it — you (or the server process) are not allowed to access this resour…
AWS: ExpiredToken — The security token included in the request is expired
Temporary AWS credentials (STS/SSO/assumed role) have expired.
Warning: session_start(): open(/var/lib/php/sessions/sess_x, O_RDWR) failed: Permission denied
PHP can’t write session files to the configured session.save_path.
HTTP 103 Early Hints
An interim response that sends Link headers early so the browser can preload CSS, fonts or scripts while the server is still buil…
HTTP 413 Content Too Large (Request Entity Too Large)
The request body is bigger than the server is willing to accept — usually a file upload over the configured limit.
WordPress: The link you followed has expired
Usually an upload (theme, plugin, media) larger than PHP’s upload/post limits — WordPress receives an empty request and shows thi…
PHP upload error 1: The uploaded file exceeds the upload_max_filesize directive in php.ini
The uploaded file is bigger than PHP’s upload_max_filesize, so PHP discarded it.
Too many open files (EMFILE)
The process hit its file-descriptor limit — sockets, files and pipes all count.
Upload error: checksum mismatch (BadDigest / Content-MD5 did not match / XAmzContentSHA256Mismatch)
The server computed a different checksum for the data it received than the one your client declared, so it rejected the upload as…
S3 EntityTooLarge: Your proposed upload exceeds the maximum allowed size
A single PUT can be at most 5 GB (and presigned POST policies can set lower limits); larger files must use multipart upload.
NotAllowedError: play() failed because the user didn't interact with the document first.
The browser blocked media autoplay with sound until the user interacts with the page.
HTTP 417 Expectation Failed
The server or a proxy can’t meet the Expect header in the request — almost always Expect: 100-continue.
WordPress: Unable to create directory wp-content/uploads/2026/10. Is its parent directory writable by the server?
PHP can’t write to wp-content/uploads — wrong ownership/permissions, a wrong upload path, full disk, or open_basedir restrictions.
HTTP 400 Bad Request
The server could not understand the request because its syntax, headers or body are malformed or invalid.
API Error: Missing required parameter (parameter_missing / "x" is required)
The API rejected the request because a field it requires — in the query string, path, headers or JSON body — wasn’t sent or was e…
API Error: Malformed JSON in request body (entity.parse.failed / JSON parse error)
The server couldn’t parse the request body as JSON, so it rejected the request before looking at any field.
HTTP 428 Precondition Required
The server requires a conditional request (If-Match / If-Unmodified-Since) to prevent lost updates, and you didn’t send one.
URIError: URI malformed
decodeURIComponent/decodeURI received an invalid percent-encoded sequence.
UnicodeEncodeError: 'charmap' codec can't encode character '\u2192'
Python tried to write text in an encoding that can’t represent some characters — common in Windows consoles and files opened with…
HTTP 415 Unsupported Media Type
The server refuses the request because the body’s Content-Type isn’t one it accepts.
WordPress: Sorry, you are not allowed to access this page.
Your user role lacks the capability for that admin screen — or the database table prefix / user capabilities are out of sync afte…
API Error: Invalid type — expected integer, received string
A request field has the wrong data type — a string where the API expects a number, a number where it expects a boolean, an object…
HTTP 416 Range Not Satisfiable
The client asked for a byte range (Range: bytes=…) that lies outside the file — usually a resumed download of a file that changed…
Cloudflare Error 1035 / 1036: Invalid request rewrite (invalid URI path / query string)
A Transform Rule (URL rewrite) produced an invalid path or query string.
Cloudflare Error 1037: Invalid rewrite rule (failed to evaluate expression)
A rewrite rule’s dynamic expression failed at runtime — for example a function received a value of the wrong type or a missing fi…
HTTP 500 Internal Server Error
The server hit an unexpected condition — usually an unhandled exception or misconfiguration in the application.
PHP Fatal error: Uncaught Error: Call to undefined function
PHP can’t find the function you called — usually a missing extension, missing include, or namespace issue.
Warning: Undefined array key / Undefined index
Your code reads an array key that doesn’t exist — common with `$_GET`, `$_POST` and decoded JSON.
HTTP 501 Not Implemented
The server doesn’t support the functionality (usually the HTTP method) needed to fulfil the request.
Chrome: This page isn’t working — example.com is unable to handle this request. ERR_HTTP2_PROTOCOL_ERROR
The HTTP/2 connection broke a protocol rule — a server, proxy or CDN sent an invalid or oversized frame/header, or closed the str…
Chrome: This site can’t be reached — ERR_QUIC_PROTOCOL_ERROR (HTTP/3 over UDP blocked)
Chrome tried HTTP/3 (QUIC over UDP) and the connection failed — usually a firewall, VPN or proxy that blocks or mangles UDP port…
Mixed Content: The page was loaded over HTTPS, but requested an insecure resource
An HTTPS page tried to load a script, API or image over plain HTTP, and the browser blocked (or warned about) it.
SecurityError: Blocked a frame with origin "x" from accessing a cross-origin frame.
Script tried to read or modify an iframe (or window) from another origin.
Script error. (at line 0) — cross-origin error details hidden
An error happened in a script from another origin, and the browser hid the details.
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