Hello SandroMelia.
You've certainly been thorough in using http://support.apple.com/kb/HT2090 and all other possibilities you have tried. Here are a few ideas you can consider if you've not already eliminated them as possible remedies:
(1) As explained in http://support.apple.com/kb/HT2411, your camera can be used by one application at a time.
To keep your Mac from launching the messages app when you connect external iSight, launch Messages and use the Messages > Preferences... Audio/Video menu command to verify that the "Open Messages when external camera is turned on" box is NOT checked (ticked).
Also verify that you have no Login Items or Dashboard Widgets trying to simultaneously use your camera(s).
(2) If you have all the same apps (including YM) on the other Macs you have used to test your external iSight, and if external iSight works properly with everything on all your other Macs, try uninstalling YM from your problem Mac. Then restart it and see if other apps work properly now. If other apps work after YM has been properly uninstalled, your problem Mac either had some problem or conflict with its YM installation. Check YM Help to see if someone there may have already found the solution to your issue.
(3) Power down all other computers on your network and check whether someone else may be using/sharing/stealing part of your wireless bandwidth. An overloaded wi-fi can slow your systems.
(4) External iSight can be rather picky about how and with what it shares your Mac's Firewire circuitry. Be sure you have no other Fiewire devices connected to your problem Mac while you troubleshoot and fix your problem.
(5) If you:
• MUST have multiple Firewire items connected to this Mac while using the camera, or
• MUST use YM on this particular Mac, and
• MUST have other camera apps simultaneously installed on this Mac, and
• cannot get things sorted any other way,
try a USB external camera instead on this Mac.
USB cams can be less finicky than Firewire cams like external iSight. Because you like external iSight's "design", you may be interested in the aGent USB webcam.
(6) Properly applying the Combo version of updates can sometimes correct software problems.
Use your Mac's Disk Utility to repair permissions and then download and apply the latest Combo Update for your Mac OS: http://support.apple.com/kb/DL1640
Repair permissions again immediately following the system restart that completes the Combo update.
Message was edited by: EZ Jim
Mac OSX 10.8.3