Hello. Actually it was a wiring problem. I've tried with both /WP and /HOLD tied to GND, then with /WP tied to GND and /HOLD floating.
This morning I saw, that /WP and /HOLD should be tied to VDD (this is noted for the Amontec JTAGkey/JTAGkey2/JTAGkey-Tiny programmer). 
After tying them to VDD everything worked fine.

Sorry for the inconvenience. 

==================================================

C:\Users\kul\Downloads\flashrom>flashrom -p ft2232_spi:type=arm-usb-tiny-h -E
flashrom v0.9.6.1-r1704 on Windows 6.1 (x86)
flashrom is free software, get the source code at http://www.flashrom.org

Calibrating delay loop... OK.
Found Winbond flash chip "W25Q64.V" (8192 kB, SPI) on ft2232_spi.
Erasing and writing flash chip... Erase/write done.

C:\Users\kul\Downloads\flashrom>flashrom -p ft2232_spi:type=arm-usb-tiny-h -w encr_firmware.bin
flashrom v0.9.6.1-r1704 on Windows 6.1 (x86)
flashrom is free software, get the source code at http://www.flashrom.org

Calibrating delay loop... OK.
Found Winbond flash chip "W25Q64.V" (8192 kB, SPI) on ft2232_spi.
Reading old flash chip contents... done.
Erasing and writing flash chip... Erase/write done.
Verifying flash... VERIFIED.


On Tue, Feb 10, 2015 at 9:24 AM, Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at> wrote:
On Mon, 9 Feb 2015 19:52:06 +0200
Kaloyan Mihaylov <kaloyan.mihaylov@gmail.com> wrote:

> flashrom.exe -p ft2232_spi:type=arm-usb-tiny-h -
> E
> flashrom v0.9.6.1-r1704 on Windows 6.1 (x86)
> flashrom is free software, get the source code at http://www.flashrom.org
>
> Calibrating delay loop... OK.
> Found Winbond flash chip "W25Q64.V" (8192 kB, SPI) on ft2232_spi.
> Erasing and writing flash chip... FAILED at 0x00000000! Expected=0xff,
> Found=0xe
> 5, failed byte count from 0x00000000-0x00000fff: 0x9ca
> ERASE FAILED!
> Reading current flash chip contents... done. FAILED at 0x00008000!
> Expected=0xff
> , Found=0x85, failed byte count from 0x00008000-0x0000ffff: 0x4bb9
> ERASE FAILED!
> Reading current flash chip contents... done. FAILED at 0x00008000!
> Expected=0xff
> , Found=0x85, failed byte count from 0x00000000-0x0000ffff: 0x4cb2
> ERASE FAILED!
> Reading current flash chip contents... done. FAILED at 0x00050200!
> Expected=0xff
> , Found=0x00, failed byte count from 0x00000000-0x007fffff: 0x30e
> ERASE FAILED!
> Reading current flash chip contents... done. FAILED at 0x002e9800!
> Expected=0xff
> , Found=0x00, failed byte count from 0x00000000-0x007fffff: 0x700
> ERASE FAILED!
> FAILED!
> Your flash chip is in an unknown state.
> Please report this on IRC at chat.freenode.net (channel #flashrom) or
> mail flashrom@flashrom.org, thanks!

Hi, that looks like erasing worked only sometimes... well, most of the
times, but not always. Have you connected all pins of the flash chip?
Is it still connected to a board? In the latter case please see
http://flashrom.org/ISP for possible culprits.

--
Kind regards/Mit freundlichen Grüßen, Stefan Tauner