This is a working e-commerce, unfortunately not have possibility to send you administration of this site for GDPR policy, ( or you sign me a regular documentation of GDPR, that i send you, for access to sensible data ) But you are able to reproduce this error on your test site, is very simple, you create one variable product with 10 colors and 12 sizes for each color.. make two or more size to 0 ( out of stock ) and see on your demo site the error, that you see on video that i send you on private area, If you don’t want to test it on your demo site, send me a temporary key of your template, in this mode i install your template to a demo site on my server for recreating this problem and send you administration and ftp access of demo site.