I encountered another strange thing in the Grid Control.
The intention was to secure the communication between the agents on AIX and the OMS. However on one machine this was working, while the other two machines where I installed the agent succeeded in putting the agent in secure mode but did never upload their data.
emctl status agent -secure
I found out that they thought that they were not in secure mode.
The solution was pretty simple (and another proof that Oracle has to find some unique way to resolve network addresses)..
In my hostfile the hosts that did not work were described as
10.1.2.3 hostname hostname.domain.com
I changed this to be
10.1.2.3 hostname.domain.com hostname
resecured everything (OMS and agents) and it then it worked.