@OlavK kappert
Please try to chat with Microsoft online to get assistance, enter your issue in that box, then click "Get Help".
Maybe you need to provide more details of your issue afther the above step, please follow the prompt on screen, and then a support case would be opened successfully as following.
You can wait patiently for a while, stay active in this chat window.
Besides, according to the link "I lost or damaged my product key", it seems that you also need to prepare a valid proof of purchase.
Hope the information could be helpful.
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