Can you provide a link to the website?

Without seeing the website, I will venture the opinion that no-one should be using IE

It has been disowned by microsoft, known security issues have been documented and distributed by Microsoft, and all of the alternatives you listed are more stable, more secure and faster.

Is this failure of the log in a recent occurrence or an on going problem you have been aware of for some time ??

Alan,

Remove cookies from internet explorer, then try again.

Hope this helps,
Best Regards