Running 0.7.53 under Win 7 x32 but noticed this happening on previous versions as I only updated to 0.7.53 today.
Started in debug mode, pasted the location for when I need it & forget where it is!
calibre, version 0.7.53
Debug mode: <p>You have started calibre in debug mode. After you quit calibre, the debug log will be available in the file: c:\users\russell\appdata\local\temp\tmplawjnh.txt< p>The log will be displayed automatically.