Just a follow-up:
I discovered it my Oxford English Dictionary that created problems, but I tried turning off 'read metadata' in the Kindle plugin as described here:
https://www.mobileread.com/forums/showthread.php?t=60645
That worked and I don't really need the metadata anyway. I still can't actually import the dictionary into Calibre - but since I've just copied it over directly, it's not a probem as such. Perhaps there's a metadata error in my file.