Developer Error Solver Hub
Instant answers and 1-click terminal commands for the most infuriating Git, Docker, Node.js, Port, and Network errors. No ads, no bloated forums, no server tracking.
Fix "Error: listen EADDRINUSE: address already in use :::3000"
Another process (usually an orphaned Node.js, Next.js, or React process) is already listening on local port 3000.
Fix "Error: listen EADDRINUSE: address already in use :::8080"
An active Apache Tomcat, Spring Boot, Jenkins, or background web server instance is already bound to port 8080.
Fix "Port 5173 is in use, trying another one... (Vite Dev Server)"
A previous Vite dev server instance (Vue, Svelte, or React) did not shut down cleanly and holds port 5173.
Fix "Error: listen EADDRINUSE: address already in use :::5000" (Flask / AirPlay)
On macOS Monterey and newer, AirPlay Receiver listens on port 5000 by default, colliding with Flask and Express servers.
Fix "Error: listen EADDRINUSE: address already in use :::8000" (Django / PHP)
A suspended Django development server or local PHP server is occupying port 8000.
Fix "SyntaxError: Unexpected token < in JSON at position 0"
Your client-side code called response.json(), but the backend API server returned an HTML error page (like a 404, 500, or Nginx error page starting with "<!DOCTYPE html>").
Fix "CORS Policy: No Access-Control-Allow-Origin header is present"
The browser blocked a cross-origin HTTP request because the target API server did not send the "Access-Control-Allow-Origin" header.
Fix "ReferenceError: window is not defined" in Next.js & React SSR
Code accessing browser-only APIs (window, document, localStorage, navigator) was executed on the server during Server-Side Rendering (SSR) or Static Site Generation (SSG).
Fix "TypeError: Cannot read properties of undefined (reading 'xyz')"
Attempting to access a property or invoke a method on a variable that evaluates to undefined or null.
Fix "npm ERR! code ERESOLVE unable to resolve dependency tree"
Conflicting peer dependencies between installed npm packages when upgrading React or Node packages.
Fix "fatal: refusing to merge unrelated histories" in Git
Git prevented a merge or pull because the local and remote repositories were initialized separately and do not share a common ancestor commit.
Fix "error: failed to push some refs to [remote]" in Git
The remote repository contains commits that you do not have locally (e.g. README created on GitHub web).
Fix "You are in 'detached HEAD' state" in Git
You checked out a specific commit hash, tag, or remote branch directly rather than a local named branch.
Fix "Conflict. The container name is already in use by container..." in Docker
A stopped or background container with the same name already exists in Docker daemon state.
Fix "Cannot connect to the Docker daemon at unix:///var/run/docker.sock"
The Docker service is stopped, Docker Desktop is not started, or the current user lacks permission to access the Docker socket.
Fix "JsonWebTokenError: jwt malformed / invalid signature"
The JWT token string was truncated, missing a dot delimiter, or signed with a different secret key than the verification routine.
Fix "Permission denied (publickey)" in SSH & GitHub
Your SSH client did not offer a public key recognized by the remote server, or file permissions on ~/.ssh/id_rsa are too permissive.
Fix "Host key verification failed / REMOTE HOST IDENTIFICATION HAS CHANGED"
The target server was re-installed or its IP address was assigned to a new machine, causing a fingerprint mismatch in your ~/.ssh/known_hosts file.
Fix "Error: certificate has expired / CERT_HAS_EXPIRED" in Node.js & cURL
The SSL/TLS certificate of the destination server has passed its validity expiration date, or your system clock is out of sync.
Fix "ModuleNotFoundError: No module named 'xyz'" in Python
The required Python package is not installed in the currently active Python interpreter or virtual environment.
Fix "FATAL: password authentication failed for user 'postgres'"
Incorrect password entered, or pg_hba.conf is enforcing md5/scram-sha-256 authentication for local sockets.
Fix "502 Bad Gateway" in Nginx & Reverse Proxies
Nginx received an invalid or nonexistent response from the upstream application server (Node.js, Gunicorn, PHP-FPM) because the backend service is stopped or listening on a different port.