That error is coming from their site at IBM.

Thanks for the response. I suppose there must be a list somewhere of all these error numbers with corresponding explanations, I don't have it.

My OS is Ubuntu 14.04.5LTS and browser is Firefox Quantum 63.0

I've now successfully logged on using another laptop with Windows 10 OS and Firefox. I now think that, owing to a recent Firefox update to my laptop, there is a conflict with certain addons which I have on my local machine. I have not managed to sort this yet, so still ongoing.

Turns out it was a corrupt cookie. Problem now solved