Overview
When using Aurea Monitor version 2021 R1 you are unable to select a site for Entry Point.
The problem is present in both the Basic and Advanced Wizard.
When clicking on the Select Site(s) button, the window appears blank (no site listed) as shown below: 
This happens in all the browsers (Chrome, Firefox, Edge).
The same Managed nodes are visible when creating the Policies.
Solution
The problem is solved after upgrading to 2021 R1 HF2 and restarting the server.