Very strange issue and I am not sure where to look next.

We have a site to site vpn using two cisco firewalls, over the public internet. We have a set of servers at the remote site. If they reboot, they lose connection to HQ. The only way to get them back is to reboot the firewall at the remote site.

So, you might be thinking: problem is with the firewall, right? Well, we happened to replace the firewall since this problem started, so if it is the firewall, it is not a hardware issue. We have also replaced the network switch they're using. And rebooting the modem doesn't have any affect.

I'm left thinking it has to be some sort of config issue on the firewall. But I have no idea what it could be. Any thoughts?