As well as including Python 3, calibre version 5 includes a new version of the Qt libraries.
There were a number of issues in the new Qt that related to Comments. I can't recall the details, I suspect you've ran into one or more of them. In the example I posted it seems to have barfed on left/right double quote entities - no idea why, maybe it wants unicode - in the scheme of things a mere bagatelle
BR