Authentication Failed: Invalid Authentication Parameters

[Eption]

I am receiving the following message when a customer tries to transfer a domain using my WHMCS:

 

> Registrar Error <

> Authentication Failed: Invalid Authentication Parameters <

 

As a newbie, I assumed this may be a 'login' connection error with Registrar however I can login to their admin area directly without a problem. I can register domains within admin area but that defeats purpose of WHMCS module connecting to Registrar.

 

Anyone out there can advise:?

 

note:

Registrar is domainbox who do not provide any support to their WHMCS module.

[they do not provide a storefront although they still advertise it on their website]