I finally solved my problem. I don't know why, but in my ossec.conf was the following section missing:
<remote>
<connection>secure</connection>
<allowed-ips>xxx.xxx.xxx.xxx/16</allowed-ips>
</remote>
This led to the described problem. Adding the section made it possible
to list the agents again (all with the duplicated-counter problem, but
i'm familiar with that).
Greets
Thomas
