Hello Holger,
Do you mean that you still have the old CCMS agents running at the same host you've installed the SAP Host Agent?
If yes, that's why the SAP Host Agent isn't working. You cannot have two CCMS agents (e.g., to sapccmsr agents) running at the same server, at the same time.
You should stop the old agents, and restart the SAP Host Agent. Once the SAP Host Agent is stable, you can then uninstall the old agents.
Regards,
Isaías