Thanks Flare for your help. My Dash-QT wallet has 3 addresses:
"main": balance is currently 1000
"masternode 1" : 0
"masternode 2" : 1000
For some reason, when I sent 1000 to "masternode 2" earlier today, they were taken from "masternode 1" instead of "main", disabling masternode 1.
There must...