Emanuele, the license activation servers will be in the format of a0.smartertools.com, a1.smartertools.com, etc. In this case I recommend trying the Reactivate button from the Settings>Licensing area, and if this does not work please try manually refreshing the license limits by clicking Activate and stepping through the activation wizard.

If this continues to fail this may indicate a DNS lookup issue or connectivity problem in reaching our activation environment. This usually stems from the server not supporting the SSL/TLS protocol suites we support, as we only accept incoming secure connections on TLS 1.2+ at this point due to the deprecation of SSL and TLS 1.0/1.1. 

Finally, if all else fails please get in touch with our teams here as we can provide you a manual activation key to get this back up and running for you, then advise you further on resolving the issue for the future. Have a good one!