You might try:
1. Uninstall Viewport
2. Reboot windows
3. Delete "C:Program FilesViewPort" entire directory if it still exists (assuming that’s where you had it installed)
4. Install viewport again (so as if new)
I’m guessing you have an old "no longer available" system font declared in one of the ViewPort xml config files. When you upgraded ViewPort, those config files might have been preserved, so a complete de-install and delete of the entire ViewPort folder would solve that.
Just make sure your own code/program files are somewhere safe! Probably they are stored safely in "My documents", but always good to check!!
Hope it works for you… best of luck!