Changing bootlogo Sony Xperia Z4 Tablet I'm trying to change to splash-screen / boot logo of my Sony Xperia Z4 tablet (SGP771)

I have already rooted my device.

With the help of the bootimg.exe tool I unpacked the boot.img - exchanged the logo.rle and repacked the boot.img.

My custom logo.rle I created with boot_screen_logo_creation_package.

However, when flashing the new-boot.img to my device (with fastboot flash boot new-boot.img) the logo is not changed. Still showing the Sony logo before the bootanimation appears.

My tablet runs the Marshmallow firmware and I'm using the corresponding AndroPlusKernel v31  (Z4T_SGP771_AndroPlusKernel_v31.zip).

Any suggestions what I am doing wrong? 

Edit: I tried already diffent tools like bootimg_tools from an xda forum, or split_bootimg.pl from the android-wiki.

The one mentioned in my question I found in several forums (wordpress, miui, dark-cyanide-devs ...) I would post the links, but I'm do not have enough reputation for posting more than 2 links...

Most of the tools I tried already did not repack the image correctly (fastboot reported "remote: dtb not found") The tool I used, mentioned in the question, seemed to work for correcly unpacking and repacking. But as I said - the logo does not change -.-
You can change bootanimation by going in root/system/media.
there you will find bootanimation.zip. open it and you will see set of images included. you can use custom images of same size and format.

it may not be accurate answer for what you asking. just some relevance.