Failed to start HTTP listener

When starting the Grid Control agent, the following error occured :
$ ./emctl start agent
Oracle Enterprise Manager 10g Release 3 Grid Control 10.2.0.3.0. 
Copyright (c) 1996, 2007 Oracle Corporation.  All rights reserved.
Starting agent ....... failed.
Failed to start HTTP listener.
Consult the log files in: /app/oracle/product/agent10g/sysman/log

If you have a look at the processes, you´ll see that the agent is still running.

[AGENT /app/oracle/product/agent10g/bin ]
$ ps -ef|grep -i em
  oracle  131100       1   0   Dec 16      - 101:31 /app/oracle/product/agent10g/bin/emagent
    root  110732       1   0   Aug 06      -  0:00 /usr/lib/errdemon
  oracle  508066  782468   0 08:57:42  pts/0  0:00 grep -i em

After killing the running agent everything works fine :
[AGENT /app/oracle/product/agent10g/bin ]
$ kill -9 131100

[AGENT /app/oracle/product/oma10.2.0/agent10g/bin ]
$ ps -ef|grep -i em
    root  110732       1   0   Aug 06      -  0:00 /usr/lib/errdemon
  oracle  508066  782468   0 08:57:42  pts/0  0:00 grep -i em

[AGENT /app/oracle/product/agent10g/bin ]
$ ./emctl start agent
Oracle Enterprise Manager 10g Release 3 Grid Control 10.2.0.3.0. 
Copyright (c) 1996, 2007 Oracle Corporation.  All rights reserved.
Starting agent ............ started.