It should be a exception that is not wrapped by a Symfony error page because there is a PayU report panel. 
This panel is the only way to check notification response but it should not contain any html. 
Just like in following picture. 
First line (with OK) is plain text response
Second one is response with exception 

![notification_panel](https://f.cloud.github.com/assets/1921950/2076188/27ba0522-8d9d-11e3-9520-843578572b7a.png)