To fix your problem GUI method :
Go into DDMS mode, click the down arrow next to the screen capture camera icon and select reset adb from the drop-down menu.
At the command line :
Open the cmd.exe command prompt. Locate abd.exe in the sdk folder ".. \ android-sdk \ platform-tools \ abd.exe"
cd <sdk directory>
(navigate to the folder above)
dir
(to make sure the abd.exe file is in this directory)
adb kill-server
(stop abd service, won't return anything)
adb start-server
(run backup abd)
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