Nexus 7 2012 Wi-Fi stucked at bootloader

Hi all, I'm really a newbie and hope you can help me with my Nexus 7 2012 wi-fi.
I tried to downgrade to 4.4.4 ( becasuse from 5.0 the tablet was unusable) following the instructions I found on gottabemobile.
I unlocked the device and everything seemed ok but when I try to write the new image with flash-all.bat I get the following errors, like the computer can't communicate with the device.

Code:

---

...
OKAY [  0.000s]
finished. total time: 0.000s
erasing 'boot'...
FAILED (remote: (Timeout))
finished. total time: 2.044s
erasing 'cache'...
FAILED (remote: (Timeout))
finished. total time: 2.028s
erasing 'recovery'...
FAILED (remote: (Timeout))
finished. total time: 2.028s
erasing 'system'...
FAILED (remote: (Timeout))
finished. total time: 2.028s
erasing 'userdata'...
FAILED (remote: (Timeout))
finished. total time: 2.028s
sending 'bootloader' (2100 KB)...
FAILED (remote: (Timeout))
finished. total time: 2.028s
rebooting into bootloader...
FAILED (remote: (Nv3pBadReceiveLength))
finished. total time: 1.014s
archive does not contain 'boot.sig'
archive does not contain 'recovery.sig'
archive does not contain 'system.sig'
archive does not contain 'vendor.img'
Creating filesystem with parameters:
    Size: 14569963520
    Block size: 4096
    Blocks per group: 32768
    Inodes per group: 8160
    Inode size: 256
    Journal blocks: 32768
    Label:
    Blocks: 3557120
    Block groups: 109
    Reserved block group size: 871
Created filesystem with 11/889440 inodes and 97309/3557120 blocks
Creating filesystem with parameters:
    Size: 464519168
    Block size: 4096
    Blocks per group: 32768
    Inodes per group: 7088
    Inode size: 256
    Journal blocks: 1772
    Label:
    Blocks: 113408
    Block groups: 4
    Reserved block group size: 31
Created filesystem with 11/28352 inodes and 3654/113408 blocks
--------------------------------------------
Bootloader Version...: 4.23
Baseband Version.....: N/A
Serial Number........: 015d2856b1400802
--------------------------------------------
checking product...
OKAY [  0.031s]
checking version-bootloader...
OKAY [  0.016s]
sending 'boot' (4994 KB)...
FAILED (data transfer failure (Unknown error))
finished. total time: 5.179s

---

If I try a fastboot devices I can see the serial number of the tablet but it is stucked in fastboot mode.
The bootloader is v. 4.23, product name is grouper, the image I'm using is image-nakasi-ktu84p (nakasi-ktu84p-factory-2c6e4d6f.tgz)
In computer management I see Android Device > Android Bootloader Interface

What did I do wrong? Have I bricked the device or it can be saved? Thanks in advance for any help. :)

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