Hi Phillipp,
This bug is already fixed in current trunk, and will be included by jbpm 4.4 release.
please refer this: https://jira.jboss.org/browse/JBPM-2859
If you cannot wait for jbpm 4.4 releasing. You could get a copy of jbpm.default.cfg.xml and replace the <repository-cache/> element to <object class="org.jbpm.pvm.internal.repository.RepositoryCacheImpl"/>