pfsense ERROR - pfctl: DIOCXCOMMIT

There were error(s) loading the rules: pfctl: DIOCXCOMMIT: Device busy – The line in question reads [ 0 ]

Here's what worked for me:
Rather than allowing my NICs to autoselect speed, duplex and flow control, I specified these on each interface, set to the highest possible setting then configured the firewall to be the master controller for these settings.

In my case that's 1000BaseT, full-duplex, flowcontrol, master.

C'est tout.
=)D