Unknown SSL protocol error in connection to [example.com]

This issue is often caused by your server failing to correctly negotiate TLSv1.1 - which is the default secure protocol used by our monitoring system and currently regarded as the most secure option for initiating HTTPS sessions.

When you see this error, your server (or an intermediary load balancer / proxy) is likely to instead require TLSv1.0 (SSLv3) and fails to correctly complete negotiation process with our TLS client.

This can be verified by using a recent versions of OpenSSL (>= 1.0.1) and disabling TLSv1.1:

openssl s_client -connect www.example.com:443 -no_tls1_1
connects successfully, whereas:

openssl s_client -connect www.example.com:443
does not.

This is often triggered by a bug in some reverse proxies / load balancers; for example some versions of "F5 Networks BIG-IP load balancer" are known to suffer from this issue.