ERR_SSL_PROTOCOL_ERROR on Windows
Why it happens on Windows
Windows routes HTTPS through Schannel, so a disabled TLS 1.2 registry key or an antivirus product that intercepts HTTPS (Avast, ESET, Kaspersky) rewrites the handshake with its own certificate and Chrome rejects it.
The fix on Windows
Turn off 'HTTPS scanning' or 'SSL/TLS protocol filtering' in your antivirus and reload. If that fixes it, add the site to the antivirus exclusion list. If not, run 'Internet Options → Advanced' and confirm TLS 1.2 and TLS 1.3 are ticked.
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
Windows 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