OK to clear things up here is my full setup:
system on mmcblk0p1 ext4
datai on mmcblk0p8 f2fs (but almost empty - used only for some data of the Linux home directory)
data on mmcblk1p2 f2fs (this is the actual data partition for android data2sd. I have the preinit script and by _that in the /boot folder, as well as a folder rootfs_data2sd with fstab.cardhu init.cardhu.rc and init.rc for data2sd. Could one of these cause the problem? I have uploaded them here
It completely works with following kernel/ramdisk/module combinations:
_that10 kernel,ramdisk,module
hundsv3.4.5 kernel,ramdisk,module
It boots, but with limitations with following combination:
hundsv3.4.5 kernel, ET ramdisk, ET module. (i.e. WLAN does not work, did not test any further)
It did not boot at all:
ET Kernel with all combinations
Thank you for investigating, I really appreciate it!