Contacted support via. Live Chat.
They advised to go (on the Dropbox Website) into the account and turn OFF 'Early releases'
Then download and install the app again from this link: https://www.dropbox.com/downloading
It's been 30 mins since I did this and haven't got the error again (it happened twice in the previous 30 minutes).
So....likely outcome is that with a recent update they've introduced a bug. So the (current) fix seems to be roll-back to the current 'safe' version...rather than being on an early release (i.e. Beta) version.
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