With a valid Json file :
PHP Warning: explode() expects parameter 2 to be string, object given in Doctrine/Doctrine/Import/Schema.php on line 381
PHP Catchable fatal error: Object of class stdClass could not be converted to string in Doctrine/Doctrine/Import/Schema.php on line 391

It's due to this line, line, in Doctrine/Parser/Json.php (#65) :
$json = json_decode($contents);

It should be:
$json = json_decode($contents, true);

Because casting the result as array will only affect the top-level element. You must use the second parameter of json_decode() to force every objects (including sub-objects) to be converted to indexed arrays.