I'm using a using a RCM6650W module and adding a simple external circuit to support Wifi and Ethernet simultaneously. Everthing works fine until I disconnect the RJ45 cable. After a few seconds of having WiFi activity the RCM6650W crashes!

It seems to be a firmware bug on the Libraries. I tried with DC10.72 and 10.72a.

Every works fine if a the Ethernet cable is connected, even if there's not activity at all on that interfase.

Can someone help me?

I would 1st check to see if the schematics show pull-up resistors on the RJ45 connections on the board. Some of these chips go nuts if inputs/outputs are floating

Yup, if you're having crashes it's likely related to the thing that has changed, i.e., the Ethernet interface (I'm assuming the cable you're unplugging is connected to a live port at the far end.) A good test would be to try plugging it into an idle switch; if the crash disappears, consider 'fooling' the Ethernet interface chip with a few strategically placed resistors..