Ok, here's the results:
Airplane Mode - didn't work.
Reset the lockdown folder - didn't work.
It starts the backup, then fails a minute or two later.
Console log shows:
9/14/15 5:02:52.278 PM AppleMobileBackup[6751]: WARNING: Backing up 1d4e77318a7a4e6910a8f46bf11aea6d736985409/14/15 5:02:58.969 PM iTunes[6155]: Entered:_AMMuxedDeviceDisconnected, mux-device:275
9/14/15 5:02:58.969 PM iTunes[6155]: Entered:__thr_AMMuxedDeviceDisconnected, mux-device:275
9/14/15 5:02:58.969 PM iTunes[6155]: tid:1d2b - Mux ID not found in mapping dictionary
9/14/15 5:02:58.969 PM iTunes[6155]: tid:1d2b - Can't handle disconnect with invalid ecid
9/14/15 5:02:58.970 PM AppleMobileBackup[6751]: AMDeviceConnect (thread 0x7fff7c78a300): Could not connect to lockdown port (62078) on device 275 - 1d4e77318a7a4e6910a8f46bf11aea6d73698540: 0xe8000084.
9/14/15 5:02:58.970 PM iTunes[5647]: Entered:_AMMuxedDeviceDisconnected, mux-device:275
9/14/15 5:02:58.970 PM iTunes[5647]: Entered:__thr_AMMuxedDeviceDisconnected, mux-device:275
9/14/15 5:02:58.970 PM iTunes[5647]: tid:1683f - Mux ID not found in mapping dictionary
9/14/15 5:02:58.971 PM iTunes[5647]: tid:1683f - Can't handle disconnect with invalid ecid
9/14/15 5:02:59.236 PM AppleMobileBackup[6751]: 6751:2088280832|DeviceLinkListener.c:_copyMobileDeviceValue| ERROR: Could not connect to attached device: This device is no longer connected. (132)
9/14/15 5:03:16.741 PM AppleMobileBackup[6751]: 6751:43008000|SocketStreamHandler.c:_SocketLogCallback| ERROR: SocketRecv (data): return of 0 bytes
9/14/15 5:03:16.742 PM AppleMobileBackup[6751]: 6751:43008000|StreamHandler.c:_ReadFunc| ERROR: Error reading data: Can't read data
9/14/15 5:03:16.743 PM AppleMobileBackup[6751]: 6751:43008000|DeviceLinkConnection.c:_DLHandlerThreadReadMessage| ERROR: Could not receive message: '(null)'
9/14/15 5:03:16.744 PM AppleMobileBackup[6751]: 6751:2088280832|DeviceLinkConnection.c:_DLMainThreadMessagePortCallback| ERROR: Error creating components from read data: 'NULL data returned from read'
9/14/15 5:03:16.744 PM AppleMobileBackup[6751]: ERROR: Connection to device lost (MBErrorDomain/200)
9/14/15 5:03:16.744 PM AppleMobileBackup[6751]: ERROR: Backup error: -10
9/14/15 5:03:16.747 PM AppleMobileBackup[6751]: 6751:43008000|SocketStreamHandler.c:_SocketLogCallback| ERROR: SocketSend (data): error 32: Broken pipe
9/14/15 5:03:16.749 PM AppleMobileBackup[6751]: 6751:43008000|StreamHandler.c:_WriteFunc| ERROR: Error sending data: Can't send data
9/14/15 5:03:16.752 PM AppleMobileBackup[6751]: 6751:43008000|DeviceLinkConnection.c:_DLHandlerThreadMessagePortCallback| ERROR: Could not send disconnect message: 'Error writing length of data'