I've never heard of F-Secure, but it should have a way to add trusted apps to it. Have you tried adding Calibre as a trusted app?
Do you also have the Calibre server checked to start each time you start up Calibre? If so, did you really intend for that option? I use the feature, but I don't have a need to leave it on all the time. This alone could be the issue. The option is under Preferences, Sharing over the Net, uncheck the box for "Run server automatically when Calibre starts".