The host manager's 404.jsp throws the following exception.

DD-MMM-YYYY HH:mm:ss SEVERE [http-nio-8080-exec-5] org.apache.catalina.core.StandardHostValve.custom Exception Processing ErrorPage[errorCode=404, location=/WEB-INF/jsp/404.jsp]
        org.apache.jasper.JasperException: Unable to compile class for JSP:

An error occurred at line: [36] in the jsp file: [/WEB-INF/jsp/404.jsp]
The method filter(String) is undefined for the type RequestUtil
33:    <h1>404 Not found</h1>
34:    <p>
35:     The page you tried to access
36:     (<%=RequestUtil.filter((String) request.getAttribute(
37:             "javax.servlet.error.request_uri"))%>)
38:     does not exist.
39:    </p>


Stacktrace:
                at org.apache.jasper.compiler.DefaultErrorHandler.javacError(DefaultErrorHandler.java:103)
                at org.apache.jasper.compiler.ErrorDispatcher.javacError(ErrorDispatcher.java:213)
                at org.apache.jasper.compiler.JDTCompiler.generateClass(JDTCompiler.java:522)
                at org.apache.jasper.compiler.Compiler.compile(Compiler.java:392)
                at org.apache.jasper.compiler.Compiler.compile(Compiler.java:362)
                at org.apache.jasper.compiler.Compiler.compile(Compiler.java:346)
                at org.apache.jasper.JspCompilationContext.compile(JspCompilationContext.java:603)
                at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:399)
                at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)
                at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329)
                at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
                at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
                at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
                at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:712)
                at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:461)
                at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:384)
                at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:312)
                at org.apache.catalina.core.StandardHostValve.custom(StandardHostValve.java:394)
                at org.apache.catalina.core.StandardHostValve.status(StandardHostValve.java:253)
                at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:175)
                at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
                at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:678)
                at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
                at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
                at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
                at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
                at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:853)
                at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1587)
                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)

---
The manager app does not have this issue, so I did a diff (output shows the relevant first few lines only):
diff -u apache-tomcat-9.0.22/webapps/host-manager/WEB-INF/jsp/404.jsp apache-tomcat-9.0.22/webapps/manager/WEB-INF/jsp/404.jsp

--- apache-tomcat-9.0.22/webapps/host-manager/WEB-INF/jsp/404.jsp       Thu Jul  4 16:23:49 2019
+++ apache-tomcat-9.0.22/webapps/manager/WEB-INF/jsp/404.jsp    Thu Jul  4 16:23:49 2019
@@ -14,7 +14,7 @@
   See the License for the specific language governing permissions and
   limitations under the License.
 --%>
-<%@ page import="org.apache.catalina.util.RequestUtil" session="false"
+<%@ page import="org.apache.tomcat.util.security.Escape" session="false"
          trimDirectiveWhitespaces="true" %>
 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
 <html>
@@ -33,18 +33,20 @@
    <h1>404 Not found</h1>
    <p>
     The page you tried to access
-    (<%=RequestUtil.filter((String) request.getAttribute(
+    (<%=Escape.htmlElementContent((String) request.getAttribute(
             "javax.servlet.error.request_uri"))%>)
     does not exist.
    </p>

---
and replaced the RequestUtil with the org.apache.tomcat.util.security.Escape
The error is gone.

br
Andreas