In the current Protege desktop 5.6.x there is a version conflict for the OWL API library:

• Protege core: 4.5.25
• rdf-library: 4.5.15

This has been reported at protegeproject#12 , but not being fixed yet.

As a workaround, I published a fixed version of rdf-library plugin as an non-official release: https://github.com/LorenzBuehmann/rdf-library/releases/tag/rdf-library-3.0.0

Steps:

• download the JAR file which gives you rdf-library-3.0.0-owlapi-fix.jar
• remove rdf-library-3.0.0.jar from $PROTEGE_HOME/plugins
• move rdf-library-3.0.0-owlapi-fix.jar to $PROTEGE_HOME/plugins
• (or just rename the downloaded JAR before)
• in any case we just have to replace the old rdf-library JAR file