Error map
Failed to complete negotiation with the server SignalR: Error: Failed to complete negotiation with the server / Failed to start the connection
.NET · layer 4, Application & API
Centre: this error. First ring: errors it is linked with (7). 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 (7)
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…
Blazor: An unhandled error has occurred. Reload 🗙
A Blazor component threw an exception; Blazor Server tears down the circuit (or WebAssembly logs to the console) and shows the re…
ObjectDisposedException: Cannot access a disposed context instance
Code used a DbContext after its scope ended — usually async void, fire-and-forget tasks, or capturing a scoped context in a singl…
WebSocket connection to 'wss://x' failed
The browser couldn’t open (or keep) a WebSocket connection.
ASP.NET Core: The CORS protocol does not allow specifying a wildcard (any) origin and credentials at the same time
The CORS policy uses AllowAnyOrigin() together with AllowCredentials(), which browsers forbid.
ASP.NET Core: The antiforgery token could not be decrypted / The required antiforgery cookie is not present (400 Bad Request)
Form POST failed antiforgery validation — the token is missing, the cookie is missing, or Data Protection keys changed so old tok…
AmbiguousMatchException: The request matched multiple endpoints. Matches: X, Y
Two controller actions or minimal API endpoints have the same route and HTTP method.
Two steps away (21)
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…
System.NullReferenceException: Object reference not set to an instance of an object
Code dereferenced a variable that is `null` — the .NET equivalent of Java’s NullPointerException.
CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views
Background code updated a View directly; UI changes must happen on the main thread.
WindowManager$BadTokenException: Unable to add window -- token null is not valid; is your activity running?
A dialog or popup was shown with a context whose Activity is finishing/destroyed, or with the Application context.
InvalidOperationException: Unable to resolve service for type 'X' while attempting to activate 'Y'
ASP.NET Core’s dependency injection container can’t build Y because one of its constructor dependencies (X) was never registered.
JsonException: A possible object cycle was detected
System.Text.Json found a reference loop (e.g. Order → Customer → Orders → …) while serialising, usually from EF Core navigation p…
InvalidOperationException: No database provider has been configured for this DbContext
EF Core created a DbContext without UseSqlServer/UseNpgsql/UseSqlite — typically because the context was newed up manually or reg…
TypeError: Failed to fetch / NetworkError when attempting to fetch resource
The browser couldn’t complete the request at the network level — CORS, DNS, offline, blocked mixed content, or the server refused…
Error: socket hang up / read ECONNRESET
The other side closed the TCP connection abruptly while your client was still using it.
Error: getaddrinfo ENOTFOUND host / Name or service not known
DNS lookup for the hostname failed — the name is wrong, doesn’t exist, or this machine can’t resolve it.
Bearer error="invalid_token": IDX10223: Lifetime validation failed. The token is expired / IDX10501: Signature validation failed
ASP.NET Core’s JwtBearer middleware rejected the token — expired (IDX10223), signature key not found (IDX10501/IDX10503), or wron…
InvalidOperationException: No authenticationScheme was specified, and there was no DefaultChallengeScheme found
[Authorize] triggered a challenge, but no authentication handler/default scheme is configured.
InvalidOperationException: Session has not been configured for this application or request
Code used HttpContext.Session, but session services/middleware aren’t registered — or UseSession runs after the endpoint that nee…
Webhook signature verification failed (Stripe, Razorpay, GitHub…)
Your endpoint computed a different HMAC signature than the one the provider sent, so the webhook is rejected.
Laravel: 419 Page Expired (CSRF token mismatch)
Laravel rejected a POST because the CSRF token was missing, expired or didn’t match the session.
Forbidden (403) CSRF verification failed. Request aborted.
Django rejected a POST because the CSRF token was missing, wrong, or the request origin isn’t trusted.
HTTP 404 Not Found
The server can’t find anything at the requested URL.
HTTP 405 Method Not Allowed
The URL exists, but it doesn’t accept the HTTP method you used (e.g. POST to a GET-only route).
Whitelabel Error Page — This application has no explicit mapping for /error
Spring Boot’s default error page — the request hit no controller mapping (404) or a controller threw (500).
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