Hi there,
I’m looking into systemd related trouble at the moment. I think this is caused by long running “asynchronous” processes (cached plugins, cached local checks or cached MRPE plugins, that run longer that a check interval is). Those keep the connection alive, resulting too many connections per source.
Can anyone confirm that (by temporarily disabling those plugins)? If that’s the case, I may be able to provide you with a workaround (and ultimately a fix, of course).