Yes, this could be a ESS or any of the piggyback boards.
Not an easy solution.
Call the Xerox Service Representative to repair the machine.

I don;t know what model you have but, with the message coming up Network controller not available there are several things that can be the problem.
On some machines, where the network is contained in a box attached to the back of the machine, often times, actually frequently the circuit boards will just work themselves loose and need to be reseated.
The, What Xerox calls the ESS, or network circuit board can be bad. On some other models the network hard drive is defective. Or the network firmware ( software that operates the network) may be corrupted and not loading. The best thing you can do is place a service call to have it resolved since that normally is not a simple issue.