What I know: person X has invited person Y. When Y clicks the link, she cannot respond. 'My appointments' is blank. She uses Internet Explorer 8.0.
Pleft seems to work normally for me in IE8.
She may have Google Chrome Frame installed. The sign in link is transformed into the default overview URL, without storing the cookie before Chrome Frame comes into action.
She has cookies disabled. Or there is some other security setting.
I have not received further debug information, and don't know what to ask for. We might want to implement something like Google Feedback in order to collect all potentially relevant data in future cases.
I expect this to have been fixed with the jQuery rewrite.