thanks for the tip. I will try a different user account, though I'll need to set up a new one.
I've noticed similar behavior on an old mac mini g4 yesterday. The mini has just been set up from scratch with OSx 10.4 install and updates. Could it be the Vuescan install has done something to the plist files? Or Filemaker? I have both these applications on each computer.
It's weird. I can see the copy in the clipboard, but can't paste. But if I open the image in Preview, then I can copy and paste...
Most of the time a plist file will be for a specific application only and wont be modified by app.
Testing in a new user will be utilizing the new user's plist files. Therefore, if the issue is not present in a new user, you are using the same applications, but different plist files (generally speaking). The problem is with the user account.
If the problem persists in the new user, by the same token, no manner of pulling out plist files will fix the problem and the issue has to be dealt with at an application level, usually involving a reinstall of the application, but not always.
If it works the same on two machines with two different versions of the OS, then it may well be the way the applications are written and not something that can be fixed with troubleshooting.
Good luck, and let me know how the testing goes. You can also try a safe boot to see if the problem persists there, but this may not allow certain components of the apps to run. Check out http://support.apple.com/kb/ht1455 for instructions.