ERR_SSL_PROTOCOL_ERROR on Mac
Why it happens on Mac
Safari and Chrome on macOS both use the system Keychain, so a stale or manually-trusted root certificate breaks every browser at once — and macOS still fails hard on servers that only offer TLS 1.0/1.1.
The fix on Mac
Open Keychain Access → System Roots, search the issuing CA and remove any manually added duplicate set to 'Never Trust'. Then run the SSL check below: if it reports TLS 1.2 or higher missing, the fix is on the server, not the Mac.
Is ERR_SSL_PROTOCOL_ERROR happening on your domain?
The SSL checker reads the live handshake: protocol versions offered, cipher suites, chain completeness and expiry. If it cannot complete the handshake either, the fault is server-side and not your browser.
Free, no signup. Runs from outside your network, which is the only way to tell a server fault from a local one.
If that didn't fix it
Mac has its own failure mode, but ERR_SSL_PROTOCOL_ERROR has a wider set of causes that apply everywhere. The most common one overall is: the server only offers tls 1.0 or tls 1.1, which every current browser has disabled.
All causes and fixes for ERR_SSL_PROTOCOL_ERROR