International Image Interoperability Framework: A community driven image framework with well defined APIs for making the world’s image repositories interoperable and accessible
In case anyone else comes along, I had the same problem using Apache 2.4.6 (Ubuntu). It had to do with apache not having permission to /usr/lib/iipimage-server. I simply copied that to /usr/share/iipimage-server and adjusted the path in iipsrv.conf and the url worked.