Always, always, always: Simultaneous captures on both client and server. I've said it before and I'll say it again: The truth is on the wire. Once you got that

Always, always, always: Simultaneous captures on both client and server. I've said it before and I'll say it again: The truth is on the wire. Once you got that, you can walk your way down with captures on intermediate devices until you catch where the packets are dropped. Once you're there it really depends on the product/OS/service/filter that's dropping the packet. If you're really experienced you can start building the house from the roof, i.e. trying by changing this or that as in your head the symptoms match other case you've seen before. If you're not, just do baby steps and you'll get there sooner than what you think. Of course you can check basic connectivity first, but when I hear "network problem" I understand that has been checked already. Edit: by the way, your problem might look like dark magic if you don't actually want to really know what's going on. Do you know how a packet is delivered to a host in your subnet? I guess not, but just so you know your machine will first try to learn the destination's MAC address (again, if it's inside the same subnet). Can you confirm you resolve your gateway's MAC address when you try to ping? Can you confirm the resolved MAC address is the correct one? There might be someone spoofing it to intercept all the traffic exiting the subnet or maybe you've got a pair of routers/firewalls and their HA setup is not working as expected. In any case, go a layer down and check that.