So, here's the full error :

lemmy-1  | 2025-07-14T00:34:32.098630Z DEBUG HTTP request{http.method=POST http.scheme="https" http.host=blog.kaki87.net http.target=/inbox otel.kind="server" request_id=a9c359f6-eb9d-4888-9ac2-55c1f5bb8e43}: actix_web::middleware::logger:
Error in response: LemmyError { message: InboxTimeout, inner: InboxTimeout, context: SpanTrace [{ target: "lemmy_server::root_span_builder", name: "HTTP request", fields: "http.method=POST http.scheme=\"https\" http.host=blog.kaki87.net http.target=/inbox otel.kind=\"server\" request_id=a9c359f6-eb9d-4888-9ac2-55c1f5bb8e43", file: "src/root_span_builder.rs", line: 16 }] }
lemmy-1  | 2025-07-14T00:34:32.098834Z  WARN Error encountered while processing
the incoming HTTP request: lemmy_server::root_span_builder: InboxTimeout: InboxTimeout

I've also been seeing this one :

lemmy-1  | 2025-07-14T00:41:00.407762Z DEBUG HTTP request{http.method=GET http.scheme="http" http.host=lemmy:8536 http.target=/api/v3/site otel.kind="server" request_id=9aab8319-3f3b-4f4e-8530-3b3633afe2f1}: actix_web::extract: Error for Option<T> extractor: IncorrectLogin: IncorrectLogin
lemmy-1  | 2025-07-14T00:41:00.427630Z DEBUG HTTP request{http.method=GET http.scheme="http" http.host=lemmy:8536 http.target=/api/v3/post/list otel.kind="server" request_id=ef746ac7-b9a3-4615-bb37-4c12f0ed0827}: actix_web::extract: Error for Option<T> extractor: IncorrectLogin: IncorrectLogin

As for the reverse proxy, I'm using Caddy with the configuration provided here, and it's supposed to pass those headers by default.

The suggested curl requests work.

I wish there was a tool that could just tell what's wrong, like https://federationtester.matrix.org/ for Matrix...