ERR_SSL_PROTOCOL_ERROR on Cloudflare
Why it happens on Cloudflare
Cloudflare's SSL mode is set to Full (Strict) but the origin serves a self-signed or expired certificate, so Cloudflare refuses the origin handshake and Chrome sees the failure.
The fix on Cloudflare
In SSL/TLS → Overview, either install a Cloudflare Origin Certificate on the origin and keep Full (Strict), or temporarily switch to Full while you fix the origin. Never use Flexible with a redirect to HTTPS — that produces a redirect loop.
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
Cloudflare 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