OWA 2012 login "almost there..."

Hi guys,
I'm facing a weird problem about OWA 2013. It's just one user who can't log into his owa. He can access his mailbox through Outlook though.
when he entres his credentials, the page remain on "almost there..." forever. Other users on same computer can log into their mailbox without any problem.

what's the reason and what shoud I do to fix it?

thanks in advanced