This application does not exist (app_id=xxx)
If you are using --oauth2 make sure that you are logged into the correct account when generating the token.
I was unable to upload to an AppEngine as appcfg was telling me :

This application does not exist (app_id=u'xxx').

I was only a developer on the AppEngine, so as I was just testing I created a new AppEngine where I was the owner but I still get the same message on a newly created AppEngine.