User defined collection error: "Collection verification failed ..." (4252407)

After adding a user defined collection and try to verify the collection, the following error comes up:

"Collection verification failed: <host name>; nested exception is: java.net.ConnectException: Connection refused"

or

"Collection verification failed: <host name>; nested exception is: java.net.UnknownHostException: <host name>"

RMI port is blocked by firewall

Please make sure that the RMI port is not blocked by the firewall and is open bi-directional.

The default RMI port for Oracle is 3723, default RMI port for SQL Server is 3235, but it can be configured.

To make sure which RMI port is set, follow the steps below:

1. Navigate to "Database Overview" dashboard
2. Tick the monitored database agent
3. Click on "Administer Agents"
4. Click on "On-demand Data Port"