Phone shows no dialogue when I try to adb backup

[Android Central Question]

Hi there,

I have an Huawei Ascend G620S-L01 with Android 4.4.4 and EMUI 2.3.
I'd like to make a full backup of the system in case something goes wrong when I try to root it later, so that I have a safe backup to restore to.

I enabled USB-Debugging in the phone's Developer Option and chose a password for the backup.

Then I got myself Android Studio and installed the Google USB Driver with help of the SDK Manager.

In the CMD I went to C:\Users\NAME\AppData\Local\Android\Sdk\platform-tools.

I connected my phone with my PC, chose the USB mass storage mode (though MTP doesn't make any differences, I tried that too). When I choose the option that the phone should just load, it won't even be recognized by adb devices.

Now, when I enter adb devices it shows my device, at least it gives some digits and letters on the left and it says "device" on the right.

Now I enter:
adb backup –all -apk -shared –f c:\backup\backup.ab
I also tried
adb backup '-all -apk -shared -f c:\backup\backup.ab'

The answer is the same:
Now unlock your device and confirm the backup operation...

But on my phone, nothing happens. No dialogue appears.

C:\backup just shows a file with 0KB named backup.ab

CMD shows again that I'm in my platform-tools folder and waits for a new order.

I hope someone can help me and guide me into making a full backup.

Thanks in advance for your support!