After the host bounce, the EM is started but get the 503 error when access the EM.
Emkey is random generate when EM repository is created. It looks like the hash encryption code.
After reviewing the EM log, it seems to be the emkey corruption issue.
This error message is somehow miss leading. It stands the emkey is at $ORACLE_HOME/sysman/config directory, but it is actually at $ORALE_HOME/$HOST_$SID/sysman/config directory. ( In my case , it is at /u01/app/oracle/product/11.2.0/panda2.localdomain_oradb2/sysman/config).
I was keep tying to import /recreate it at wrong location and without success.
In the end, I have just drop and recreate the repo
emca -deconfig dbcontrol db -repos drop
emca -config dbcontrol db -repos create
After repository is recreated, I got the agent unreachable in the EM.
From emctl status agent, I saw the message Collection Status : Disabled by Upload Manager
I run to test the agent, it seems fine.
After, I pork around, it is because the agent upload drive is full. I just clean up the space and it works fine.