'Access denied' even after permission granted

This issue partially explains things: #789484: Confusing documentation for $node argument in hook_access()/hook_node_access()

So basically, if the $op is 'create', then $node could be just a string containing the node type.

To overcome this, we can copy the $type from node_content_access:

function node_content_access($op, $node, $account) {
  $type = is_string($node) ? $node : (is_array($node) ? $node['type'] : $node->type);
  // rest of the node access code...
}

Commited.