[DiapDealer]

That would be my guess too. Checking that box in the prefs should do the trick.

EDIT: I just noticed the part about "open with in calibre". If you're using the inherent Open With feature of calibre, I would expect no trouble (though I've never tested it myself -- a python program opening a C++ program that's launching an external python process is always going to be fraught with peril IMO), but if you're using the older, third-party Open With calibre plugin, then who knows? It had a long history of problems with Sigil in general. *shrug*