Hello
I have found the problem. 
I have updated the board like this:
khadas@Khadas:~$ sudo apt update
khadas@Khadas:~$ sudo apt full-upgrade
khadas@Khadas:~$ sudo do-fenix-full-upgrade
khadas@Khadas:~$ sync
khadas@Khadas:~$ sudo reboot
Now the /boot/dtb/kvim3_linux.dtb was updated.
But in u-boot, the file dtb.img is loaded.
And this dtb.img file is older than the kvim3_linux.dtb.
So i copied the kvim3_linux.dtb to dtb.img and now it works.
I have decompiled both dtb’s.
Old One
i2c@1c000 {
compatible = “amlogic,meson-g12b-i2c”;
status = “okay”;
New One
i2c@1c000 {
compatible = “amlogic,meson-g12b-i2c”;
status = “disabled”;
How could it happen, that the dtb.img was not updated?