Hi,
The server log returns this error:
[09-Jun-2015 08:24:07 UTC] WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 3 for query SELECT p.* FROM wp_lqfj_posts AS p
INNER JOIN wp_lqfj_term_relationships AS tr ON p.ID = tr.object_id INNER JOIN wp_lqfj_term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
WHERE p.post_date < '2015-04-04 22:42:39' AND p.menu_order = '26' AND p.post_type = 'product' AND p.post_status = 'publish' AND tt.taxonomy = 'category' AND tt.term_id IN () made by require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), include('/themes/joyn/woocommerce/single-product.php'), woocommerce_get_template_part, wc_get_template_part, load_template, require('/themes/joyn/woocommerce/content-single-product.php'), previous_post_link, get_previous_post_link, get_adjacent_post_link, get_adjacent_post, apply_filters('get_previous_post_where'), call_user_func_array, cpto_get_previous_post_where
[09-Jun-2015 08:24:07 UTC] WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 3 for query SELECT p.* FROM wp_lqfj_posts AS p
INNER JOIN wp_lqfj_term_relationships AS tr ON p.ID = tr.object_id INNER JOIN wp_lqfj_term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id
WHERE p.post_date > '2015-04-04 22:42:39' AND p.menu_order = '26' AND p.post_type = 'product' AND p.post_status = 'publish' AND tt.taxonomy = 'category' AND tt.term_id IN () made by require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), include('/themes/joyn/woocommerce/single-product.php'), woocommerce_get_template_part, wc_get_template_part, load_template, require('/themes/joyn/woocommerce/content-single-product.php'), next_post_link, get_next_post_link, get_adjacent_post_link, get_adjacent_post, apply_filters('get_next_post_where'), call_user_func_array, cpto_get_next_post_where
I have no idea what this means but seems to have something to do with the theme.
.