Unlocking Bootloader: "command write failed" after most fastboot commands.

So 10 hours ago I decided I wanted to root my phone to install some apps and maybe even a cool ROM.
I read that the first step is to unlock the bootloader. I have trying my hardest to do this first step for 10 hours.
I've read 4-5 different tutorials twice on xda.

My phone is:

HTC One A9(hiae), bought in Sweden

hTc download mode

*** LOCKED ***
htc_hiaeuhl PVT S-ON
LK-1.0.0.000
RADIO-1.1.U00018.1@60122.4
OpenDSP-7.8.2.00197-8952_1228
OS-1.56.401.70
Jan 22 2016....

I have tried the normal way through htcdev.com even had the Unlock_code.bin at one point. But I had to reinstall all drivers twice to get it.
I have tried using the WinDroid Toolkit, no better than the manual way.

I got my hard earned token ID through both ways.

I find that every time I want to use fasstboot to send over a file i.e. "fastboot flash recovery twrp.img" hTc download mode is stuck at Start download, like I need to confirm it but can't because the command failed. The same goes for the Unlock_code.bin, can't send it over.

Code:

---

C:\adb>fastboot devices
HT5AVBE04449    fastboot

C:\adb>fastboot flash unlocktoken Unlock_code.bin
target didn't report max-download-size
sending 'unlocktoken' (0 KB)...
FAILED (command write failed (Unknown error))
finished. total time: 0.003s

---

I can get past this next problem temporarily by reinstalling all drivers, tedious.

Code:

---

C:\adb>fastboot oem get_identifier_token
...
FAILED (command write failed (Unknown error))
finished. total time: 5.002s

---

Possible problems:
hTc download mode: I need to accept the download before fastboot give up, instant.
Win10: Bad OS for this?
USB connectors: Not enough power? Some other USB devices don't work here normally.
HTC Sync Manager: I've not bother installing the software.

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