Vault Login Failed: The server uses a protocol that is not supported by this client.

Installing CruiseControl.NET on a new server and ran into an issue with CruiseControl.NET pulling from the Vault server. I found the following error in the CruiseControl.NET Log:

[PROJECT:ERROR] Vault Standard Login Failed Twice.:The server uses a protocol that is not supported by this client.

Vault Standard Login Failed: The server uses a protocol that is not supported by this client.

The problem turned out to be an incorrect version of the Vault CruiseControl.net Plug-In. I had accidentally grabbed the latest version (7.0) but the Vault server is running an older version of Vault (6.1). The older version could be found on the Previous Vault Downloads page. Installing the correct version of the Vault plugin to match the Vault server solved the problem.