SSL Certificate Errors and Solutions: 2026 Comprehensive Guide

Fighting SSL Certificate Errors

SSL certificate errors are one of the most common problems that website owners encounter. These errors both negatively affect visitor experience and harm your SEO performance. In this comprehensive guide, we will discuss the most common SSL errors and their solutions in detail.

1. ERR_CERT_AUTHORITY_INVALID (Invalid Certificate Authority)

This error indicates that the browser thinks your SSL certificate is not issued by a trusted authority.

Possible Causes:

• Using a self-signed certificate
• Incomplete installation of intermediate certificates
• The certificate chain is incomplete
• An outdated or unrecognized certificate authority

Solution Steps:

1. Download the CA Bundle file from your certificate provider
2. Configure the certificate chain in the correct order on your web server
3. Verify your certificate chain with the SSL Checker tool
4. Restart the server and repeat the test

2. NET::ERR_CERT_DATE_INVALID (Date Invalid)

Your SSL certificate has expired or has not started yet. This is one of the most easily solved errors.

Possible Causes:

• The certificate has expired
• Server time set incorrectly
• The validity start date of the certificate has not arrived yet

Solution Steps:

1. Check the certificate expiration date: openssl x509 -enddate -noout -in certificate.crt
2. Renew the certificate if it has expired
3. Synchronize server time with NTP server
4. Activate automatic renewal systems

3. Mixed Content Error

Your page is loaded over HTTPS, but some resources (images, scripts, CSS) are called over HTTP. This causes security warnings in browsers.

Solution Steps:

1. Review mixed content warnings in the browser's developer console (F12)
2. Update all resource URLs to HTTPS
3. Bulk modify HTTP links in database
4. Add Content Security Policy (CSP) header: upgrade-insecure-requests
5. Redirect HTTP → HTTPS with .htaccess

4. ERR_SSL_PROTOCOL_ERROR (Protocol Error)

There was a problem with the SSL/TLS handshake process.

Possible Causes:

• Using older TLS versions (TLS 1.0, 1.1)
• Errors in server SSL configuration
• Incompatible cipher suites

Solution:

1. Enable TLS 1.2 and TLS 1.3 support on your server
2. Disable older TLS versions
3. Use powerful cipher suites (ECDHE, AES-GCM)
4. Get your configuration to A+ with the SSL Labs testing tool

5. ERR_CERT_COMMON_NAME_INVALID (Domain Mismatch)

The domain name in the certificate does not match the address the browser is trying to connect to.

Possible Causes:

• www and non-www versions not defined in the certificate
• A certificate was installed for the wrong domain
• The subdomain is not covered by the certificate

Solution:

1. Check the SAN (Subject Alternative Name) fields of the certificate
2. Make sure both the www and non-www version are in the certificate
3. Request a new certificate with correct domain information if necessary

6. HSTS Error

HTTP Strict Transport Security error occurs when the SSL certificate of a previously HSTS active site is faulty.

Solution:

1. Fix or renew SSL certificate
2. Check your status in the HSTS preload list
3. Keep HSTS header times short in the test environment

Tips to Prevent SSL Problems

• 🔔 Set a reminder 30 days before certificate expiration
• 🔄 Activate automatic renewal systems
• 🔍 Check your site regularly with SSL Checker
• 📋 Verify the certificate chain on each installation
• ⚡ Increase both security and performance using TLS 1.3
• 🛡️ Configure HSTS and CSP headers correctly