You can downgrade to 2.66 by uninstalling 2.67 and reinstalling 2.66, previous versions are here ==>>
Previous calibre releases
You will NOT lose libraries or settings by downgrading.
But wait a while, Kovid will probably be around in a bit and he may have better suggestions.
Assuming a Libre file is an ODT file from Libre office, you could try saving as DOCX and converting that, I've converted DOCX's on 2.67 with no problems, but they are not image intensive.
BR