Hi Pavel,
same problem here. If it helps this is the error trace:
[error] 812#812: *13528 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Error: Call to a member function getTimestamp() on string in [...]wp-content/plugins/atum-stock-manager-for-woocommerce/classes/Inc/Helpers.php:1658
Stack trace:
#0 [...]wp-content/plugins/atum-stock-manager-for-woocommerce/classes/Inc/Ajax.php(444): Atum\Inc\Helpers::update_product_data(665, Array)
#1 [...]wp-includes/class-wp-hook.php(286): Atum\Inc\Ajax->update_list_data('')
#2 [...]wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array)
#3 [...]wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#4 [...]wp-admin/admin-ajax.php(100): do_action('wp_ajax_atum_up...')
#5 {main}
thrown in [...]wp-content/plugins/atum-stock-manager-for-woocommerce/classes/Inc/Helpers.php on line 1658" while reading response header from upstream, client: ..., server: ..., request: "POST /wp-admin/admin-ajax.php HTTP/2.0", upstream: "fastcgi://unix:/var/run/php-fpm/[...]_nginx.sock:", host: "www.[...]", referrer: "https://www.[...]/wp-admin/admin.php?page=atum-stock-central"
Best regards,
scadmon