Are you running Parallel server?
No

Do you have the PROGRAM parameter specified in your listener.ora?
No

check logs, listener.log in server side and sqlnet.log in client side
I cannot see error msg in listenner.log (as far as I can interpret). I can see error msg in sqlnet.log but I don't think it is the root cause of the problem, because the error has been happening long before the present problem. The first 2 lines of the error are :
"Fatal NI connect error 12631, connecting to:
(DESCRIPTION=(LOCAL=YES)(ADDRESS=(PROTOCOL=beq))) "

sometimes 3113 leave trace files in udump and alertSID.log, check those as well
I cannot see error msg in alter log file.

also are you using same user to login?
I don't think it is related to the user. I get the error after I run svrmgrl (even before trying to connect by a particular user).

There's a parameter "..LISTENER_TIMEOUT.." in the listener.ora, check it out the value, and increase it if necessary.
I cannot see _TIMEOUT entry in the listener.ora file.

-----------------------------------------------
As I can connect to the Oracle server from another client machine, I presume the problem is happening because of some client problem on the Oracle server. Do you think reinstalling the client on the Oracle server is a good idea? BTW this is a test server.