Symptoms
Error when attempting to open connection to K2 server through K2 API (connection.Open):
SAML Token could not be verified. Please ensure your config is setup correctly
Diagnoses
The call is being made from a custom page hosted on SP2013 using Claims Based Authentication
Resolution
Issue resolved after following https://help.k2.com/onlinehelp/k2blackpearl/icg/current/webframe.html_claims_authentication_configuration.html