By default ISG proxy would generate a default hostname in Management UI - Configuration - Authentication - Windows Domain for Proxy to be joined to domain. The default hostname would be SG-<serial no> (e.g SG-3614552111).
As Enterprise license uses the same serial number across all virtual proxy, other proxy using the same license would have the same generated hostname causing Active Directory (AD) to use the same machine account when attempting to authenticate. Authentication has seen to be failing intermittently as a result of this.
1. Remove all references to the realm
2. Delete the realm that is pointing to the Windows domain
3. Leave the domain
4. Delete the ISG computer object in AD
5. Select the "Hostname" radio button (seen in above screenshot) and enter the hostname of the proxy
6. Click "Join" to join the domain.