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.
The fix I found was to add the parameter
--no_cookies:This can happen when you upload AppEngine applications from different Google accounts on the same computer.