Hi Christos,
>> I am getting the following vCloud Error: Single sign-on failed for this organization.
Sounds like you are preparing a SAML transaction in this case.
As this error is coming from vCloud, so they can shed some light on root cause.
Upon finding the root cause from SP ( vCloud in this case ), try to check what Gluu Server missing or what type of cases are not being matched to prepare a successful SSO.