A fix has been merged and will be included in the next patch release: Agent receiver crashes on IPv4-only Checkmk servers (regression introduced by werk 15416)
In the meantime, you can use the fix mentioned above. Alternatively, you can also patch etc/init.d/agent-receiver using our fix in case you have a mixed (IPv4-only/IPv6) setup:
15409 FIX Agent receiver crashes on IPv4-only hosts (regression intro… · tribe29/checkmk@95d0d1d · GitHub
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