Error Debug Library

70 field-tested fix guides. Each one: symptom, root cause, ordered fix, prevention.

certbot

• Certbot Renewal Failing: 4 Checks Before It Expires
• Certbot HTTP-01 Fails Behind a Proxy: Two Options

cloudflare

• Cloudflare Error 1101: Worker Threw an Exception
• Cloudflare 522: Origin Connection Timed Out
• Cloudflare 521: Web Server Is Down
• Cloudflare Error 1000: DNS Points to Prohibited IP
• Cloudflare Error 1020: Access Denied by Firewall Rule

cors

• CORS Preflight Failing: The Header Checklist
• CORS: "credentials mode is include" — The Three-Header Rule

cron

• Cron Job Not Running: The Silent Failure Checklist
• Cron Job Running at the Wrong Hour: Timezone Traps

dns

• "DNS Not Propagating": Read the Authoritative Server

docker

• Docker Exit Code 137: OOMKilled, Explained
• Docker Exit Code 126: Permission Problem Fix
• Docker "Permission Denied" on /var/run/docker.sock: Fixed
• docker: "bind: address already in use" — Find the Squatter
• docker build "no space left on device": Reclaim It Safely
• docker network "already exists / overlaps": Subnet Collision Fix
• Docker Container Restarting: Crash Loop vs OOM vs Policy

docker-compose

• Docker Compose Connection Refused on Localhost: The Real Reason

elasticsearch

• Elasticsearch Read-Only Block: Flood Stage Disk Watermark

env

• Your .env Is Not Loading: The Five Usual Reasons

fail2ban

• fail2ban Banned You: Unban Without Disabling It

git

• Git Detached HEAD State: Safe Recovery

grpc

• gRPC "Deadline Exceeded": Where the Time Goes

gunicorn

• Gunicorn "[CRITICAL] WORKER TIMEOUT": What It Really Means

helm

• Helm UPGRADE FAILED: Another Operation In Progress

http2

• ERR_HTTP2_PROTOCOL_ERROR: Find the Misbehaving Layer

kafka

• Kafka Consumer Group Constantly Rebalancing

kubectl

• kubectl Talking to the Wrong Cluster: Context Discipline

kubernetes

• Kubernetes CrashLoopBackOff: Systematic Fix
• Kubernetes ImagePullBackOff: Causes in Order of Likelihood
• Kubernetes Node Not Ready: Troubleshooting Guide
• Kubernetes PVC Stuck Pending: Read the Events First
• Kubernetes Ingress 404: The Three Mismatches
• ConfigMap Changed But Pods Did Not Notice
• Kubelet Cert Expired Overnight: Check the Clock First

mongodb

• MongoDB Connection Failures: SRV String, Allowlist, and DNS

mysql

• MySQL Error 1040: Too Many Connections
• MySQL 1045 "Access Denied for User": Three Real Causes
• MySQL Slow Queries: Turn On the Right Logging

nginx

• NGINX 502 Bad Gateway: 5 Checks That Find It Fast
• NGINX 504 Gateway Timeout: Diagnosis From the Logs Out
• NGINX 499: Client Closed Request (What It Really Means)
• NGINX 413 Request Entity Too Large: Uploads Blocked
• NGINX 400 "Request Header Or Cookie Too Large"
• NGINX 500 "Internal Redirect Cycle": Break the Loop

nodejs

• Node.js ECONNRESET: Where It Comes From and What to Do
• Node.js "Reached Heap Limit": FATAL ERROR Fix

npm

• npm ERR! code ERESOLVE: Peer Dependency Conflict Fix

pip

• pip "externally-managed-environment" (Ubuntu 23.04+): The Right Fix

postgres

• Postgres Connection Pool Exhaustion: Finding the Leak
• Postgres "Deadlock Detected": Kill the Root Cause
• Postgres Query Slow: Read the Plan, Then Index

prometheus

• Prometheus Targets DOWN: Scrape Debug in 4 Steps

python

• ModuleNotFoundError: You Installed It Somewhere Else

react

• React Hydration Mismatch: Dates, Random, and Browser APIs

redis

• Redis Connection Refused: 3 Usual Suspects
• Redis NOAUTH: Authentication Required

sqlite

• SQLite "Database Is Locked" (SQLITE_BUSY): What Actually Fixes It

ssh

• SSH "Permission Denied (publickey)": Fixed for Good
• SSH "Connection Refused" vs "Connection Closed": Different Bugs

ssl

• SSL Handshake Failed: It Is Usually the Chain

systemd

• Systemd Service Restart Loop: Diagnosis Path

terraform

• Terraform "Error Acquiring the State Lock": Safe Unlock

ubuntu

• Ubuntu Out of Memory: Add Swap the Safe Way
• Ubuntu DNS Failing: systemd-resolved in Four Commands
• Disk Full But du Finds Nothing: Deleted, Still Open

ufw

• ufw enable Locked Me Out: Recovery and Proper Order

wireguard

• WireGuard "Handshake Did Not Complete": The Four Checks

NinjaOps publishes free engineering guides. Some outbound links are affiliate links: they cost you nothing and support the site.

Last reviewed 2026-10-02 · NinjaOps SEO grid · DevOps blog