API Authentication Error

I am experiencing an issue with test code that up until relatively recently has been working for a number of years.

I initiate the authentication process via the API and am directed to the Login page. I enter the credentials and then instead of being redirected to the page to click the Authorise button, I end up back on the home page, with an error message "The security code provided was incorrect. Please try again."

What "security code"? Surely this isnt referring to the "access token"?

And the strange thing is that it only happens on the second iteration - it always works the first time.

There's nothing I can debug at my end because control is not returned to my app between hitting the Login button and expecting to see the Geni authorization page.

Can someone please help?