I’d like to update this thread now that we better understand what went wrong.
We have error-handling logic in Lightroom CC that watches for the rare case where a customer might have chosen to delete their online catalog while Lightroom was running. Early on Oct. 6, one of our gateway servers rebooted, and for a very brief time it sent out a signal that provoked this error dialog in our app. Anyone who had left Lightroom CC running during that time saw this alarming message when they came back to their computer. No catalogs or photos were ever deleted. It was a bogus warning, and we sincerely apologize for the scare.
We’ve now made this bit of error handling more robust and changed the wording. And the server team is investigating their reboot protocol to make sure they come back online gracefully.