Error! Check Activation. - Why am I getting this?

Hi,

    I'm getting this error message when I'm trying to add a book to the library.  I'm already registerd and the PC has been activated - I've added books to the library and copied them to my Kobo before without a problem.

I've checked the firewall settings and it's not blocking Adobe Digital Editions.

Any ideas what I can do?

Thanks in advance.

Liz