[A5_UL] Unable to flash RUU

Hey guys, first post, but I'm kinda stuck right now.

My phone:
*** Software Status: Modified ***
*** Relocked ***
A5-UL PVT SHIP S-ON
HBOOT-3.19.0.0000
RADIO-1.101.111011.20.0123
OpenDSP-v27.2.2.00546.0311
OS-
eMMC-boot 1536MB
Aug 7 2015,14:02:57.0

Okay, I tried flashing a custom ROM (InsertCoin A5_CHL & UL) and it installed properly, without any issues. But after rebooting it wouldn't connect to my Google Account. It just keeps checking and checking.. and after a good five minutes it states that vDM client ist shut down.
So, I followed this guide and tried going back to Stock RUU, but every time I enter the fastboot command

Code:

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adb reboot fastboot

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my phone just restarts, it won't enter fastboot mode.

I already tried to enter manually [Bootloader -> Fastboot] and the command

Code:

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fastboot flash 0P9CIMG.zip .zip

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is working. It's sending the zip file to my phone, but every time it starts to write the actual file I get an error message.

Code:

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C:\ADB>fastboot flash zip 0P9CIMG.zip
sending 'zip' (1033121 KB)...
OKAY [ 46.667s]
writing 'zip'...
(bootloader) signature checking...
FAILED (remote: signature verify fail)
finished. total time: 122.015s

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Stock RUU I'm using is this one.

So, my question..
How can I get back to Stock RUU? Is there any other way to make it happen?

Download from this link