Weird behaviour from the MacBook Air, strange permissions requests from Droplr & Dropbox at bootup, Outlook account kept disassociating its password from the Mail app, OneDrive app wasn't recognising its own login details although it was fine in a web browser.
A litany of utter lunacy.
Bit the bullet & restored to an 8-day-old Time Machine backup.
Eventually, and after a restart to settle things, it's all working fine again & I've recovered 6 GB of free space.