"This should never ever happen, Magix!" This is not Magix support. This forum is a user-to-user peer group and the moderators are not Magix employees either. Authorization issues are handled by the Magix Customer Service Sales Team.
That said, SF must occasionally be connected to the internet to verify MP3 and other third-party encoder licensees. Have you tried saving in the PCM format <.wav> .
Is your serial number displayed accurately in "Help> About Sound Forge Pro". You can try resetting SF back to the initial installation defaults. Otherwise, uninstall Pro 12 with a third-party uninstaller, one that removes leftover folders, registry entries and such the Windows uninstall leaves behind.. then re-install and authorize with the PC connected to the internet.