Due to a change introduced this morning to our BusyBox source, to flash a new nightly moving forward you must do a one-time manual process.
Download tonight’s (2/14) nightly, unpack the zip and use fastboot to flash the boot.img.
fastboot flash boot boot.img
This will allow you to resume the normal update process. Sorry in advance for the hassle, but this was a necessary change for how mounts are handled in BusyBox. As Sony devices use a combined boot and recovery.img (no separate recovery partition), fastboot is the only way to workaround this latest change.