This error can occur due to the following reasons:

1. The target host's IP address is not reachable.

2. The Vembu Integration Service (VIS) is not available or not running on the target host.

Cause 1:

Ensure the following:

1. Verify that the target host is reachable.
2. Confirm that the Target Host IP Address and password are correct. If these details have changed, update the host information on the BDRShield Server to establish a successful connection.
   

Cause 2:

The BDRShield Backup Agent, known as 'BDRShield Integration Service (BIS),' is a crucial component for backup operations, and it is installed on the target host via the BDRShield Server. If BIS is unavailable due to accidental uninstallation or removal, follow these steps:

1. Reinstall BIS on the target host.

2. Ensure that BIS is running on the target host. The BDRShield Server cannot connect to the target host for backup operations if BIS is stopped. To confirm BIS's status on the target host, follow these steps:

For Windows:

1. Press the 'Win + R' keys on your keyboard to open the 'Run' window.
   
2. Type ‘services.msc’ and click OK.
   
3. Check the status of the service ‘VembuIntegrationService'.

For Linux:

1. Open the Terminal and run the command: `netstat -anp | grep 42005`
2. 42005 is the port used by Vembu Integration Service (VIS). Ensure that it is in use.