@Galaxia Thanks for a long reply! I tried to flash on ubuntu 16.04(in Vbox) this morning, and found that USB port name changed after rebooting into ubports. In tab "USB Setting" of Vbox, it was "LGE Nexus 5[0232]" after normal booting and "Google Android [0100]" in fastboot mode while "LGE Hammerhead on Ubports [0232]" after rebooting into Ubports during flashing. My solution is adding a new filter rule about Ubports in "USB Setting" of Vbox, ending up with a successful flashing.

However, I have no idea about the root cause of this failure too. Maybe OS does mastter?! Up to now, I have played around with UT from stable and devel_stable channel, both of which suffers several problems. I will switch to devel_rc-proposed at ur advice

@mixmoto Actually, my nexus 5 is never locked

Hi, guys! I tried to flash ubuntu touch(maintained by Vojtech Bocek) into nexus5 but failed. Following https://developer.ubuntu.com/en/phone/devices/installing-ubuntu-for-devices and https://devices.ubports.com/#/hammerhead, I have installed ubuntu-device-flash and phablet-tools. After enabling usb debugging and rebooting into fastboot mode, I executed the command:

ubuntu-device-flash --server=http://system-image.ubports.com touch --channel=ubuntu-touch/stable --device=hammerhead --bootstrap

Nexus 5 did reboot into CWM Recovery but it still complained "Failed to enter Recovery". I have searched for solutions online for a while but found nothing help. Did I missed anything here? Any suggestion will be appreciated!

OS: Deepin GNU/Linux 2015
PPA: 
    # ubuntu sdk F141B5F0C7122F9B
    deb http://ppa.launchpad.net/ubuntu-sdk-team/ppa/ubuntu xenial main 

    # snappy dev tools F1831DDAFC42E99D
    deb http://ppa.launchpad.net/snappy-dev/tools/ubuntu xenial main 

    # phablet tools 5C5364B55E51A24C
    deb http://ppa.launchpad.net/phablet-team/tools/ubuntu xenial main

Err: 
    root@dp572:~# ubuntu-device-flash --server=http://system-image.ubports.com touch --channel=ubuntu-touch/stable --device=hammerhead --bootstrap

    2017/01/07 10:47:20 Device is |hammerhead|
    2017/01/07 10:47:21 Flashing version 12 from ubuntu-touch/stable channel and server http://system-image.ubports.com to device hammerhead
    2017/01/07 10:47:50 Waiting for device to enter recovery mode ...
    Failed to enter Recovery