This application does not exist (app_id=xxx)
Tried to upload to the app this morning (first time i've tried since Friday) and it just worked (first time...!) 
No idea what the issue was as I haven't done any work on this over the weekend so everything should have been the same as it was on Friday.
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.