Browser
ERR_SSL_PROTOCOL_ERROR The TLS handshake failed
What it means
The browser and the server failed to agree a TLS (HTTPS) session. The reasons vary: a broken certificate setup, a protocol mismatch, a middlebox interfering, or a server that is not actually speaking TLS on that port.
Where you usually see it
HTTPS sites after a bad certificate change, some public Wi-Fi filters, and servers that still offer only old protocol versions.
What to try first
Check that the address really begins with the host you intended. Try again later in case a deploy is mid-way. If you run the site, check that the certificate and the chain are current and that the service is speaking TLS on port 443. A second browser can tell you whether the fault follows the machine or the site.
When to stop
This is not fixed by turning off security warnings. If a browser tells you the connection is unsafe, do not send a password through it while you "just test".