That’s a big jump in version numbers. I would recommend that you upgrade one version at a time, fix all broken tests, work to remove as many deprecation warnings as possible, do some “lite user testing” and then move on to the next version. I would do this with both Ember and Ember-data.
I know that probably sounds very tedious. But as someone who’s been using Ember since the 0.9.x days, I can tell you that it will be much easier and you’ll end up with a lot fewer regressions. And when you do encounter unexpected things like this it will make googling for a solution much easier (since you know which version caused the problem).
With regards to the specific issue, unfortuntaly it could be any number of things. Obviously check for errors in your console. My first guess is it’s not locating your application template. You might try
App.__container__.lookupFactory('template:application') in your console. If that comes back null/undefined, then the problem is clearly with the resolver or the location of your file.