Now I have different feedback from FF and IE.
On FF I get the classic error page, on IE I get a blank page with only a "LOADING" message (in IE I get also correct name for the tab: "avast! Account", see attached screenshot).
Anyway, I've now tried to use my phone connection and the website works fine, so it's a problem related to my office's network settings. It's the first website that I find blocked in some way and the different behaviour between IE and FF it's really strange...