Check your trace file and see what it says. It could be because that you have double quotes around GLOBAL_DBNAME

To fix this, shutdown the intelligent agent dbsnmp_stop,

shutdown the listener

remove all the double quotes around the Global_dbname in the listener.ora file under $ORACLE_HOME/network/admin

Clean your old intelligent agent files

restart the listener

restart the Intelligent agent


Hope this would fix the problem for you.
Sam