Protege 5 has switched to Owl-Api version 4 after beta17. This shifth required major refactoring in the RMDB to still be compatible with Protege 5 beta 18 and so on. RMDB now is using the same Owl-Api version 4 to be compatible with the most current Protege 5 build (https://github.com/protegeproject/protege.git) This version has been tested on Protege 5 beta 24 - SNAPSHOT. Building: Generate the jar with command line >mvn clean package and copy the generated at.ainf.querydebugger.jar together with ca.uvic.cs.chisel.cajun.jar from the project's root directory into the plugins directory of you Protege 5.