ERR_SSL_PROTOCOL_ERROR on Chrome
Why it happens on Chrome
Chrome disabled TLS 1.0/1.1 outright and enforces certificate transparency, so it fails where older browsers still connect.
The fix on Chrome
Open chrome://net-internals/#hsts and clear the domain's HSTS/security state, then Settings → Privacy → Clear browsing data → Cached images and files. If it still fails, the server is offering a protocol Chrome rejects.
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
Chrome 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