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Are there any ERROR, WARN or Exception messages in the log?
There are different reasons to see an empty vault. One possibility is that due to a sync conflict, a new (empty) vault root was created, see Empty Vault, but still allocates Space. If you are using WebDAV, afaik, if one file is corrupt in a directory listing, the while directory is displayed empty. And also there may be a problem with google filestream, i.e. that even if the files are demanded, they are not dowloaded. (but this would be the first time we hat this ticket on MacOS.)