FortiClient 5.0.7 - FortiGate 5.2.1 - Windows 8.1

[Blacktip]

Post installing FortiGate 5.2.1 on our Firewall, when trying to connect using the FortiClient  5.0.7 client, the error message “Unable to logon to the server. Your user name or password may not be configured properly for this connection. (-12)” appears.

Most references I found were related to not using "special" characters in either your username or password, which didn't apply to us.

The fix for us was ensuring that you had TLS 1.1 & 1.2 enabled in IE.!!!