Is there a zombie ArmoryDB.exe process on your system? Are you automating ArmoryDB or running it manually? If manually, is there some other process listening on ArmoryDB's port (9001)?
It must have been something like that; rebooting the computer has fixed it.
I run Bitcoin core manually, and when that has caught up, run ArmoryQt manually. I guess that starts ArmoryDB automatically.
Thanks for the help. Much appreciated.