This is a syntax error at the closing brackets of {{ app_root }}:
If a YAML line starts with {{ you have to quote the line, because YAML thinks it may be a hash.
It’s not really a bug and is documented on the YAML syntax page (though that, I realize, is a bit off the beaten track).
Ah, right there at the bottom of http://www.ansibleworks.com/docs/YAMLSyntax.html in the Gotchas section. Thanks.