dblink server closed the connection unexpectedly - Exception

Hello,

I got the following exception from dblink query:
==================================================
sql>...
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!>

Note: "sql>" is changed to "i>"

When re-start database server, the error disappear automatically.

Could someone let me what may cause this exception please?

Found an online doc saying:
===============================
2.3 Port-Scanning Via Remote Access
When a connection attempt fails, dblink throws an sqlclient unable to
establish sqlconnection exception including an explanation of the error.
Examples of these details are listed below.
SELECT * FROM dblink_connect(�host=1.2.3.4
port=5678
user=name
password=secret
dbname=abc
connect_timeout=10�);

Port is open
DETAIL: server closed the connection unexpectedly This probably means
the server terminated abnormally before or while processing the request

In a real example, how do I interpret this? Still what may cause "Port
is open" exception?

Thanks alot!

--
Emi