I posted the other day that Avast Free was suddenly blocking my Outlook 2007, and that it only worked when I disabled the Avast shields for 10 minutes.
I've since noticed that Mozilla Thunderbird was also blocked, but unlike with Outlook, when I disable the shields for 10 minutes, I get a message like your one.
I'm starting to think the solution is to switch to another Antivirus program.
Regards,
Renato