Gotta love meaningless error msgs (-50) and apparently Apple has not discriminated between either Mac or PC, both seem to be getting these error -50 messages.
As to your question, the default setting for DeviceBackupDisabled is actually "False".
This enables the automatic backup of your iPhone/iPod Touch/iPad. When you set it to "True" you disable the automatic backup...
Seems like reverse logic to me, why not name this 'DeviceBackupEnabled', oh well...
I came across this setting back in the day when I still had iOS 4 on my iPhone 3G. I had changed this setting to "True" because a backup of the iPhone with each sync would literally take an hour or so...
I even wrote a small AppleScript application to toggle this setting between "True" and "False" easily so I could occassionally make a backup, just not all the time...
Too bad that the iOS Upgrade process in iTunes does not properly check for this exception and instead throws this generic error, maybe this will get addressed in a future version of iTunes, we can only hope.
So people who run into this thread looking possible solutions to error -50, if you have this on a Mac, there is a good chance the "defaults write com.apple.iTunes DeviceBackupsDisabled -bool false" solution may work, however since the error is generic, there may be a different reason you're getting this...