Some item options or their combination are not currently available in Magento 2.3

In a store the products are importing via a connector with clients ERP.
Variations (size attribte only) and products that are out of stock are not visible in the store.
When we try to add a product in the cart Magento randomly gives error “Some item options or their combination are not currently available.” to some variations even they are in stock and with available quantity.

For example we have a jacket with sizes 48, 50, 52, 54, 56
We can buy only two of them (48, 50 for example) and all the other are showing the error.
This happends almost to all products
The website is https://onnetgr.com

Thanks in advance for any help

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