In the absence of other suggestions (summer holidays?) you could try a 'safe boot' which will clear a lot of caches among which the little offender may be lurking. Press and hold the shift key at startup or restart before the chime. Don't forget to restart normally afterwards. The longer the restart takes the more likely a lot of fixes are active, so be patient!
For the record, I eventually just did an ordinary boot (‘unsafe boot’??!) and the Mac does not nag me anymore.
Must remember to switch it off and switch it back on again before posting something in the discussion group
Safe boot just ignores any start-up associated with non-Apple software and clears all your caches. Clearing caches removes any potentially corrupted data but does put you back in that desperately slow state you get after an OS update.
It's "safe" because Apple's software is always perfect. /s
Richard I 