And one more push.
Still no update.
In the meantime I found more code mistakes.
For example the javascript function flat_responsive_post_nav() in the file /inc/template-tags.php contains a mistake. The output of the code looks like this in the frontend:
← %Title
%Title →
The correct output should be the actual title of the previous or next post instead of a percent sign and the word title.
I corrected the code myself. It sits within the link generating brackets and has to do with your code trying to implement the text-domain. I removed it:
<?php
previous_post_link( '
%link
', '
%title' );
next_post_link( '
%link
', ' %title
' );
?>