There is a known issues for On-Premises CRM environments that have a multiple server setup (CRM is on one server and DB is on another server) and their SPN is not configured correctly.
This results in the Admin Settings page showing up blank.
The best solution to this issue is to contact Microsoft Dynamics Support, and enlist their support in configuring the SPN correctly.
A work-around to the problem for most environments:
If this work-around does not solve the problem, contact Microsoft CRM Support to investigate the SPN for your environment.
Once the SPN issue is resolved, the Isolation Mode setting can be returned to Sandbox.