java.lang.IllegalStateException: Form too many keys

strange log from our server...

11:36:50.753-----------> ServletHandler.doHandle: /webui/zkau;jsessionid=ioszr2q047gs1tyu34enmgbep [3880]
java.lang.IllegalStateException: Form too many keys
at org.eclipse.jetty.util.UrlEncoded.decodeUtf8To(UrlEncoded.java:526)
at org.eclipse.jetty.util.UrlEncoded.decodeTo(UrlEncoded.java:641)
at org.eclipse.jetty.server.Request.extractFormParameters(Request.java:370)
at org.eclipse.jetty.server.Request.extractContentParameters(Request.java:302)
at org.eclipse.jetty.server.Request.extractParameters(Request.java:256)
at org.eclipse.jetty.server.Request.getParameter(Request.java:825)
at org.zkoss.zk.au.http.DHtmlUpdateServlet$3.getDesktopId(DHtmlUpdateServlet.java:733)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:531)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:485)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:494)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:800)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:587)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)