SSL protocol error
ERR_SSL_PROTOCOL_ERRORThe TLS handshake fell apart before a secure connection could form, often from a protocol mismatch or a broken server config.
What it means
ERR_SSL_PROTOCOL_ERROR is a catch-all for a handshake that failed at the protocol level. The browser and server tried to agree on how to secure the connection and couldn't finish. Unlike the certificate-specific errors, this one points at the negotiation itself: an unsupported TLS version, a malformed certificate file, an interfering proxy or antivirus, or a server that's serving plain HTTP on the HTTPS port.
When it happens
It tends to appear after a server change: a TLS upgrade that dropped support for the version a client still uses, a botched certificate install that left the file malformed, or a misconfigured virtual host. On the client side, security software that intercepts HTTPS, a corrupted local SSL cache, or an out-of-date browser can all trigger it. If every visitor sees it, the server is the place to look; if only you do, start local.
How to fix it
- Run an SSL test against the server to check the TLS versions and certificate are valid and well-formed.
- Make sure the server still offers a TLS version the client supports, and that the certificate file isn't corrupted or missing its key.
- On the client, clear the browser's SSL state, update the browser, and temporarily disable any HTTPS-scanning antivirus or proxy to see if it's interfering.
- Confirm nothing is serving plain HTTP on port 443, which produces this error instantly.
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Related errors
ERR_SSL_VERSION_OR_CIPHER_MISMATCHThe browser and server share no common TLS version or cipher suite, so they can't agree on how to encrypt the connection.
NET::ERR_CERT_AUTHORITY_INVALIDThe browser can't trace the certificate back to a trusted authority, which is what happens with self-signed certs or a missing intermediate.
ERR_CERT_WEAK_SIGNATURE_ALGORITHMThe certificate was signed with an algorithm now considered too weak to trust, such as SHA-1.