Nitro Software officially ended support for version 10 some time ago. What does this mean for you?
: Nitro officially advises upgrading to a modern version (such as Nitro PDF Pro 14) to ensure you have the latest security patches for modern vulnerabilities [17, 22]. Troubleshooting Activation If you have a valid serial number for version 10: patch nitro pro 10
Because the activation servers for version 10 are no longer active, standard serial numbers often trigger a "connection issue" error. Nitro Software officially ended support for version 10