I found the error…
I was declaring variables like this…
global $infinite_loops_single_pagination, $related_enable_sidebar, $ads_sidebar_height;
$related_enable_sidebar=get_field('related_enable_sidebar','options');
$ads_sidebar_height=get_field('ads_sidebar_height','options');
$infinite_loops_single_pagination=(bool)get_field('infinite_loops_single_pagination','options');
… outside a function.
