Hi Cyril ,
Questions related to Azure DevOps are best asked over on https://developercommunity.visualstudio.com/search?space=21, I would suggest posting your question there.
As far as related to your issue Ensure that the application registration or MI which is acting has the service connection between azure Devops and azure has the necessary permissions to access the Azure Key Vault. Verify that the application has been granted the appropriate permissions (e.g., Get and List permissions) on the key vault where the certificate is stored.
Kindly check and accept answer if it helps, Thanks!