This is the error in the iChat preferences. Ralph as usual is the expert here. derbbre solution solved the issue for other users. “Try:
Open Terminal (it’s in the Utilities folder, in the Applications folder).
In Terminal type or paste this command on a single line:
defaults write com.apple.iChat newbwdup 300000
Or if that doesn’t work try Evans suggestion “That’s it. It worked for me to deleting com.apple.ichat.plist in my home library folder. Thank you so much.” It is in the “Hard Drive name/Users/(your Account Name)/Library/Prefernces” folder.