Are the netsus and the mac mini server currently on the same subnet during your testing? If they are, is the netsus running and serving out images at the same time as the mac mini? If they are, can you try booting to your mini with the netsus turned off?
I ask because I just ran into a similar issue. Our netsus acts like a "Master" netboot server while it and my mac mini server are both running and serving out netboot images in the same subnet. I can see images from both servers when I boot my client test computer while holding down the option key. But when I select the image from my mini, the netsus acts like a master and tries to make the client boot from it using the image that was selected from the mac mini. Since the client can't find the image, it resorts to loading the login screen from the internal hard drive.
It took some packet capturing from Apple to get this figures out for us.
Food for thought
