I've just checked and my VAT number is shown as "validate" on the link you send me. I've tried on AppDrag with and without the FR but both are not working.
Thanks for your advice, unfortunately in my case the products are courses and I cannot add the delivery process.
But I think, I found my mistake. In the shop configuration, there is an option to add the European VAT automatically which was selected. I've removed it and made few test and it seems working now.