Can’t connect Oracle 11g2 Database to Job Scheduler Agent on client/application server

Running 11g2 database on one Linux server and installed Oracle client and job scheduler agent on separate Linux (application) server sitting next to it. The connection betweeen them worked fine for years.

Then I changed the host names for the both the db and app servers.

Database Server Host Name Change:

host4.old_domainname.com --> host4.new_domainname.com

Application Server Host Name Change:

host3.old_domainname.com --> host3.new_domainname.com

I didn’t change the IP addresses, nor the Oracle database SID or service name. I updated the schagent.cont file HOST_NAME appropriately, as well as /etc/hosts files on both servers, rebooted the servers and brought up the DB, listener and schagent, but get an error:

ORA-12537: TNS:connection closed
ORA-06512: at "SYS.DBMS_ISCHED", line 4966
ORA-06512: at "SYS.DBMS_ISCHED", line 6898
ORA-29273: HTTP request failed
ORA-06512: at "SYS.UTL_HTTP", line 1130
ORA-12541: TNS:no listener
ORA-06512: at "SYS.DBMS_SCHEDULER", line 3975
ORA-06512: at line 1

Looking for hints how to troubleshoot/fix.