@mdonner Perhaps some pictures of your setup might help, from the Routing / Gateways and Gateway Groups pages.
Pfsense isn't really involved in traffic between devices on your LAN, so it's really strange that you would see this happening?!
Apparently you are able to access pfsense UI from a client, so do you have internet access when on the failover connection? And do the servers have internet access?
Are you using VLAN's to separate your servers from your clients??