Try to replace your Guvnor with 5.3.0-SNAPSHOT. To get that war, go to https://repository.jboss.org/nexus/index.html#nexus-search;quick~guvnor-distribution-wars and navigate to 5.3.0-SNAPSHOT directory, the select the first in list (guvnor-distributions-wars-5.3.0-SNAPSHOT-jboss-as-5.1.war), in the "Artifact Info" tab then click on the Download button.
Plase this war in your $jbpm_installer/lib directory and in $jbpm_installer/build.properties change
drools.guvnor.version=.....
to
drools.guvnor.version=5.3.0-SNAPSHOT
and in $jbpm_installer dir run
ant clean.demo
ant install.demo
ant start.demo
for the new version to be picked up. irrc there were some changes to the Guvnor REST API after the jBPM5.1 release that fixed these issues with posting new content.
Hope this helps.