Zywall blocks clients

Those ZyXEL errors are not something that blocks access, having seen those on occasion, and disabling push notifications — which were not in use locally — avoided triggering that logging. That traffic does get dropped, and the rest of the traffic continues. They're also between the server and the firewall, and not something I'd expect to effect the clients.

As for the problem, everything is working — except for two computers? That would tend to rule out the server and the firewall, barring a firewall-specific rule that's getting triggered for the specific computers or some factor common to those two computers. I'd focus would be on what those two computers might be doing that triggers the connectivity problems, or if there's some sort of networking error involving those two computers, some sort of subnet-specific configuration issue or wiring-configuration issue or multi-path routing or such. If Wi-Fi, switch those clients to wired, check the Console.app logs for relevant errors, etc. If there's more than one network link active in Network Preferences on the problematic clients, make sure the preferred link is first or disable all secondary links. Also check the DNS server settings, as I've seen wonky DNS configurations or DNS problems cause outages, so test that and test by pinging by host address and not DNS name. DNS timeouts can make remote access appear very slow, too.

As for the errors with those messages now arising with Facebook, check back with the folks at ZyXEL.