Hi Andreas, reinstall checkmk. but persist the problem.
Install checkmk on site, nothing docker.
output site
Addresses: ip public
Tags: [address_family:ip-v4-only], [agent:cmk-agent], [criticality:prod], [networking:lan], [piggyback:auto-piggyback], [site:metamonitorb], [snmp_ds:no-snmp]
Labels: [cmk/site:metamonitorb]
Host groups: check_mk
Contact groups: check-mk-notify
Agent mode: No agent
Type of agent:
Process piggyback data from /omd/sites/metamonitorb/tmp/check_mk/piggyback/metamonitorb
PING only
Services:
checktype item params description groups
persist the problem 
configured tls, dns, nothing
In the checkmk server have install the agent?.
In my host run cmk-agent-ctl pull
output
ERROR [cmk_agent_ctl] Failed to listen on TCP socket for incoming pull connections.
Error with IPV6:
Address in use (os error 98)
Error with IPV4:
Address in use (os error 98)