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Expression isYPSubContext is undefined on line 13, column 6 in default/header.htm.
The problematic instruction:
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==> if-else  [on line 13, column 1 in default/header.htm]
 in include "header.htm" [on line 1, column 1 in default/message.htm]
 in include "${templateName}/message.htm" [on line 7, column 9 in exception.html]
----------

Java backtrace for programmers:
----------
freemarker.core.InvalidReferenceException: Expression isYPSubContext is undefined on line 13, column 6 in default/header.htm.
	at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:124)
	at freemarker.core.Expression.isTrue(Expression.java:145)
	at freemarker.core.IfBlock.accept(IfBlock.java:80)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.MixedContent.accept(MixedContent.java:92)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.Environment.include(Environment.java:1375)
	at freemarker.core.Include.accept(Include.java:155)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.MixedContent.accept(MixedContent.java:92)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.Environment.include(Environment.java:1375)
	at freemarker.core.Include.accept(Include.java:155)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.IfBlock.accept(IfBlock.java:82)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.MixedContent.accept(MixedContent.java:92)
	at freemarker.core.Environment.visit(Environment.java:196)
	at freemarker.core.Environment.process(Environment.java:176)
	at freemarker.template.Template.process(Template.java:232)
	at net.jforum.exceptions.ExceptionWriter.handleExceptionData(ExceptionWriter.java:115)
	at net.jforum.JForum.handleException(JForum.java:285)
	at net.jforum.JForum.service(JForum.java:205)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at net.jforum.util.legacy.clickstream.ClickstreamFilter.doFilter(ClickstreamFilter.java:59)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
	at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:880)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1601)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.base/java.lang.Thread.run(Thread.java:834)