I was unable to upload to an AppEngine as appcfg
was telling me :
<This application does not exist (app_id=u\'xxx\').
I fixed this problem by editing my app.yaml file to have the correct application name. In the app.yaml file where it says 'application:[insert app name here]', I had to change the [insert app name here] part. I'm not sure why the name was wrong, but I do remember toying around with it before while attempting to fix a different bug. The way I figured out the proper name was to go to my applications in my GAE admin console and look to see what the app's name was there. Hope this helps anyone else that runs into this same problem :)