Hi @oceanic, sorry about the trouble.
The client ID (an identifier for our app generated by Google for authorization purposes) was mistakenly deleted by one of our developers. To address this we immediately created a new release with a new valid client ID for Google Drive sync to work, since all existing versions were built with the old client ID and were unable to sync with Google Drive.
We no longer support versions of macOS (née OS X) lower than 10.10. We do still have Codebook version 3.0.9 available to download, that was the last version to support 10.8 and 10.9. However, because it was built with the old Google Drive client ID (the one that was deleted), it will no longer sync to Google Drive.
If you cannot update OS X to 10.10 you won’t be able to run later versions of Codebook on your Mac past version 3.0.9 and resume syncing with Google Drive. You could choose another sync method, Dropbox or Desktop WiFi might be the way to go.
By the way, the version on my iPhone now seems to be unable to find this location for desktop syncing, too.
I’m not sure exactly what the problem is, but we do have a trouble-shooting guide for WiFi sync: