Please confirm the following steps.
1. Confirm Admin Console upgraded to 5.9.5. Click on Defender OU | click on Defender drop down menu | click on About
2. Try adding the new DSS server to the Access Node with ADUC on a different domain controller.
3. Defender OU | Security Servers OU | Does the new DSS object exist here for the new server?
Installing the DSS Server does not create the DSS object in AD automatically. you would have to create it | right click in Security Servers OU and select New | Defender Security Server | Add the name and IP of the new DSS