Ajax error 500

Hi,
I have a problem with updating products in your plugin. Everytime when I change the stock status and POST to server, it will freeze my screen and in console I’ll get error Internal Server Error 500.

/wp-admin/admin-ajax.php:1 
Failed to load resource: the server responded with a status of 500 (Internal Server Error)

I use version 1.2.6.

I tried to test it on three separate pages WP shops, this problem occurs on all tested websites by me.

Thank you for any help.