Looks like the upgrade to Portable 0.9.19 just broke Quality Check in a big way.
It disappeared from my main toolbar, and while it still shows as 'Installed', I can't configure it; it says 'You must restart calibre before you can configure the Quality Check plugin', and I can't add it back to the toolbar either.
HELP!
P.S. Yep, going back to 0.9.18 fixes it without doing anything else.