I was having the same problems on my iMac, but by accident I managed to solve it. When first starting Calibre, it asked for a folder to store my ebooks. After choosing a folder that's on my network it gave me this:
Just now I started it again and instead of a network folder I stuck a new folder on my desktop and chose that. No errors, Calibre works perfectly.
Maybe the cause of the error is that the mac version of Calibre can't handle folders that are on Windows network machines?