Primary and Secondary domain is Windows Server 2012 R2.
TLS 1.2 is enforced on both.
Thank you for the provided details.
How would installing a service in the secondary domain work with our other domains? Can it manage just the one domain?
An instance of Adaxes service can be configured to manage only specific domains, but the approach will not work in your environment because of load balancing. The thing is that information about registered domains is replicated between the instances of Adaxes service that share common configuration and an instance of Adaxes Web Interface can connect to the service where the domain should not be managed. In this case, management of other domains will not be possible for the users who should be able to do it. On the other hand, if all your domains are managed via an instance of Adaxes service that shares common configuration with other ones and resides in the secondary domain, the domains management will be performed the same way as it works now. So, there is no reason to limit the domains managed via an instance of Adaxes service.
Before installing an instance of Adaxes service in the secondary domain, you can try to install only an instance of Adaxes Web Interface and check whether the issue persists.