This issue is not completely investigated yet.

First some background. There is this hack in order to connect a Zabbix server to a remote Zabbix proxy in passive mode. This connection is tunneled through a SSH connection which itself is tunneled through a HTTP-Proxy. While this is indeed pretty ugly, it works surprisingly well most of the time.

Now that connection appears to get sometimes somehow affected in a way that it (tunnel process) is still available, but Zabbix history synchronization (and possibly more) freezes.

When killing the SSH tunnel process then, Zabbix immediately continues history synchronization and recovers quickly. Well, at least when disabling all enabled actions - but that's a different story (...*cough*...ZBXNEXT-3086 ).

Quite some time ago I experienced something similar. After having applied the first patch attempt of ZBXNEXT-2074, to the (btw, very same) proxy, that proxy hung itself up and Zabbix server also stopped processing completely.

It would be nice, if Zabbix server can become resilient against remote component/connection issues of any kind.

PS: I was struggling whether this is rather a ZBXNEXT or ZBX. Currently I find it more a bug. Nevertheless, feel free to move it, if you disagree