Illegal installation - Why does it say I'm running an illegal SMF installation

There are a few explanations. One may be that you have tried to remove the copyright from your theme in violation of our license. Please replace it.

Another reason is that the language files for your theme have become corrupt. Re-upload the "languages" directory for the theme you are receiving the error on. If you receive the error on all themes, try re-uploading the default themes "languages" directory.