For the last few months, TimeMachine has been failing for me between my Macs and a QNAP TS-659 NAS I have. The NAS is running firmware 4.0.3, and I would consistently get the error “Cannot connect” when trying to connect. I was able to work around this with the following command run manually: Running that in a terminal window allowed me to then use the GUI to select that disk for backups and things started rolling again.