Hi Pavel,
Thanks for you quick reply.
We use WP 5.0.3, WC 3.5.3, PHP 7.0.32 and ATUM 1.5.2.1.
There are errors in the http log:
[Wed Jan 16 11:34:01.105235 2019] [:error] [pid 23024] [client x.x.x.x:18674] PHP Fatal error: Uncaught Error: Call to a member function getTimestamp() on string in /home/website/domains/website.nl/public_html/wp-content/plugins/atum-stock-manager-for-woocommerce/classes/Inc/Helpers.php:1658\nStack trace:\n#0 /home/website/domains/website.nl/public_html/wp-content/plugins/atum-stock-manager-for-woocommerce/classes/Inc/Ajax.php(444): Atum\\Inc\\Helpers::update_product_data(3036, Array)\n#1 /home/website/domains/website.nl/public_html/wp-includes/class-wp-hook.php(286): Atum\\Inc\\Ajax->update_list_data(”)\n#2 /home/website/domains/website.nl/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array)\n#3 /home/website/domains/website.nl/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array)\n#4 /home/website/domains/website.nl/public_html/wp-admin/admin-ajax.php(99): do_action(‘wp_ajax_atum_up…’)\n#5 {main}\n thrown in /home/website/domains/website.nl/public_html/wp-content/plugins/atum-stock-manager-for-woocommerce/classes/Inc/Helpers.php on line 1658, referer: https://www.website.nl/wp-admin/admin.php?page=atum-stock-central