Our dear user ozolli kindly gave me access to his build box and I successfully identified the issue. He is using Debian GNU/Linux stretch/sid.
LibreELEC is using host tools for formatting EXT4 partition. And Debian is using newer version of ext2 filesystem utilities
which supports some new filesystem feature called metadata_csum. But e2fsprogs 1.42.13 used by LibreELEC doesn't understand that. And populatefs fail at the end.
There are 3 possible solutions:
1.) If whole build is already done then mke2fs should disable some features (64bit,metadata_csum).
The line should look like this in file scripts/mkimage:
2.) For new builds e2fsprogs package should be bumped to 1.43.3
file packages/sysutils/e2fsprogs/package.mk
3.) remove feature metadata_csum from /etc/mke2fs.conf
for advanced users only!