@vlastaw I’m not sure if you figured this out but I just hit this issue. I had HASSOS setup (in a VM, running on proxmox) for about a year now. About 6 months ago I added a 2nd nic which physically connects to my IoT VLAN.
Earlier today I found my HASS randomly can’t connect to anything. I ssh’ed in, and sure enough can’t ping it. It looks like the metric for the IoT VLAN NIC somehow became lower than the normal LAN NIC, which resulted in nothing being accessible (because nothing in that VLAN can access the rest of my network).
To “Fix” this, I simply removed the DNS & Gateway IP from my IoT VLAN NIC (so it only has IP & subnet - previously it was set to DHCP). This is fine for me since this interface is only for talking to devices in that subnet, the system should really use the other NIC for everything else.