fckeditor浏览服务器的上传图片时，Internal Server Error(500)，上传没有问题。

The server didn't send back a proper XML response. Please contact your system administrator.

XML request error: Internal Server Error (500)

严重: Servlet.service() for servlet Connector threw exception
java.lang.NoSuchFieldError: DIRECTORY
 at net.fckeditor.response.XmlResponse.setFolders(XmlResponse.java:185)
 at net.fckeditor.response.XmlResponse.setFoldersAndFiles(XmlResponse.java:228)
 at net.fckeditor.connector.ConnectorServlet.doGet(ConnectorServlet.java:149)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
 at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
 at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
 at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
 at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
 at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
 at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
 at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:849)
 at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
 at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:454)
 at java.lang.Thread.run(Thread.java:619)

详细的ERROR，请看图片。这是什么引起的呢？