I solved this problem by replacing the poi-jars.. Export2XLS and export2XLSX works fine. Export2pdf dosn't work, but it is not relevant for me..
BiServer 8.2-CE and POI 3.17 are not compatible.
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-3.16.jar
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-3.17.jar_original
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-ooxml-3.16.jar
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-ooxml-3.17.jar_original
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-ooxml-schemas-3.16.jar
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-ooxml-schemas-3.17.jar_original
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-scratchpad-3.16.jar
/.../pentaho-server/tomcat/webapps/pentaho/WEB-INF/lib/poi-scratchpad-3.17.jar_original