Protection Error - This application cannot continue because another application with an identical serial number is already running on another computer on your network

There have been recent posts appearing about your problem. I'm not sure if anyone has found a solution.

FCE was abandoned by Apple years ago, and they are currently making Operating Systems without regard to how it impacts FCE.

countrynewsman wrote:

I uninstalled and reinstalled FCE...didn't work.

That is what I would try - a fresh install - but your need to use FCS remover (from the same software source as Preference Manager) to make sure you get all traces of your existing installation off the drive prior to doing the installation:

http://www.digitalrebellion.com/fcsremover/

If you use FCS Remover, and reinstall the other problem may be that Apple has apparently removed the Software Upgrade to FCE 4.0.3.

So unless you have a 4.0.3 install disk, or you hadn't upgraded past the version number of the install disk you do have, you legacy projects may be tagged as being created with a later version than you have and not open.

Hope it works out for you.

MtD

FCP7.03 Protection Error, out of the blue!