Thanks guys, I could able to fix the same kind of issue with my instance too as there were multiple instances on the same server and the KEY was conflicting with other instances. I change the KEY to EXTPROC00 and restarted the listener and it worked.
Your resolution helped me to fix the issue.
Wow, this actually worked for me as well. The OS is AIX, which is the Oracle certified OS.
I've verified all the permissions on the $ORACLE_HOME/network/*, $ORACLE_HOME/bin/lsnrctl, and /var/tmp/.oracle. Both listener.ora and tnsnames.ora had "(KEY=EXTPROC1)", but listener started up after I change only the listener.ora file with "(KEY=EXTPROC0)"