Multi Inventory doesn't work well in combination with the WP Multilang plugin. It fails to read the Main Inventory meta-data (stock amount, etc...)
Undefined offset: 0 in /staging/2/wp-includes/meta.php on line 492
I did some research and I found the culprit. The WP core function get_postmeta() has an inconsistency when all meta for a post is being queried while $single == true. (See https://core.trac.wordpress.org/ticket/45273#no0 )
An easy workaround is replacing the following line in the get_meta() function of atum-multi-inventory/classes/Models/Inventory.php:
$meta_data = get_metadata( $meta_type, $inventory_id, $meta_key, $single );
$meta_data = get_metadata( $meta_type, $inventory_id, $meta_key, $single && ! is_null( $meta_key ) );
I would appreciate it if you implement this easy fix in the next release of Multi Inventory.