SSLHandshakeException: PKIX Path Building Failed (Untrusted Certificate)

What the log looks like

javax\.net\.ssl\.SSLHandshakeException: PKIX path building failed: sun\.security\.provider\.certpath\.SunCertPathBuilderException: unable to find valid certification path to requested target
sun\.security\.validator\.ValidatorException: PKIX path building failed: sun\.security\.provider\.certpath\.SunCertPathBuilderException

How to fix it

1. Turn off local proxies, VPNs, or network accelerators (such as Watt Toolkit, Steam++, or Fiddler) and temporarily disable the 'HTTPS Scanning' or 'Encrypted Web Traffic Inspection' feature in your antivirus software.
2. Update your system Java to a modern release such as Adoptium Temurin 21 (including the ARM64 build if you are on an Apple Silicon M-series Mac), as older Java 8 builds lack current Cloudflare and Let's Encrypt root certificates.
3. Check your operating system's Date & Time settings and enable automatic time synchronization, since an inaccurate system clock will cause valid SSL certificates to be rejected.
4. If you must use an antivirus HTTPS filter or corporate/school proxy that installs certificates into your OS, make Java trust your system store by setting the JAVA_TOOL_OPTIONS environment variable to -Djavax.net.ssl.trustStoreType=WINDOWS-ROOT on Windows (or -Djavax.net.ssl.trustStoreType=KeychainStore on macOS) and restarting the program.
5. For Fabric, Quilt, orForge setup issues, install the mod loader directly through a modern launcher like Prism Launcher instead of running standalone .jar installers.

Check your own log