Hello @Arnaud_Synetis ,
Thank you for posting here.
I noticed that this post has the same content as another post you posted, URL:
https://learn.microsoft.com/en-us/answers/questions/64571/signing-an-externally-generated-csr-with-ad-cs-sta.html
If you can confirm that the content of the two posts are the same, then in order to ensure the efficiency of support, we will archive this post and reply and assist you in another post. Thank you for your understanding.
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Daisy Zhou