img_auth.php rejects to serve diagram outputs under images/. Is there any solution besides disabling img_auth.php? Or is the output location configurable, like Extension:Widgets does?
Topic on Extension talk:Diagrams
Sorry, I've only just seen this post! I've opened GH#61 to track this, and will try to get it fixed soon.
This can be fixed with a configuration change, now detailed in the installation instructions.