I got an array being returned from an ACF field after moving onto my production web server. The cause is, I suspect, the bad migration procedure. I re-imported several times the WP database via XML export files one on top of another and again.
As I use Timber/Twig, my code causing errors is {{post.front_page_quick_order|apply_filters('the_content')}} where the ACF field is front_page_quick_order.
Making var_dump I saw clearly that front_page_quick_order was an array of strings.
Searching in the WP database via PHPMyAdmin for front_page_quick_order gave me 25 entries in the wp_postmeta table.
The quick fix 1 was: {{post.get_field('front_page_quick_order')|apply_filters('the_content')}} – just use the standard ACF function to get the field instead of Timber/Twig shorthand syntax.
The fix 2 (the one I prefer): install WordPress WP-Sweep plugin and clean up the entire DB. Then return the code to the initial state {{post.front_page_quick_order|apply_filters('the_content')}}.
NB: I tried WPOptimizer with no effect. After WP-Sweep 25 entries become 3.
