Kustom wallpaper is vanishing 20 seconds after reboot


I hope I did not oversee the final solution in this forum but nothing I saw solved my issue.

I have a Toscido 10.1'' tablet running Android 9, Nova 6.2.12 and KLWP v3.45b7815.

Everything is set up fine, even after reboot I can see the background wallpaper for 15-20 seconds. Then suddenly the wallpaper is replaced by a system background.

After checking the internet I already deactivated the battery optimization for KLWP, KLWP Pro Key and Nova.
I also tested setting the KLWP wallpaper with KLWP itself and with Android/Settings/Background.

Still the behaviour is always the same, reboot brings everything as it should and then after some 20 seconds I get a standard background.

Is this the right forum or do I have to go to reddit or create a ticket?
Please help me out, I am setting up this tablet for my mother to create a really clean (not to confuse)
desktop and also to create Huge icons. If it does not work after reboot the story gets unusable...