I called the Nuance support and yes, they are indeed experiencing troubles with activation services. The company in the USA is aware of the problem, and they hope to get things back to normal within the next few days.
Meanwhile, they advised me to use DNS in "trial mode" (I had just switched to a new PC...)
[Modifié le 2017-06-29 13:19 GMT]
Brilliant. Thanks for the info.