I am using wso2is-5.11.0 along with wso2am-4.1.0 and I use an API with proper mediation to call the admin service 'RemoteUserStoreManagerService' and the operation 'isExistingUser'. The API works fine but sometimes it responds with the following error.

{
    "faultstring": "Access Denied."
}

In the IS log, the following exception could be seen.

[51de4e9b-63c9-4a01-8a57-ea80c9caefdb] ERROR {java.lang.Class} - Access Denied. Failed authorization attempt to access service 'RemoteUserStoreManagerService' operation 'isExistingUser' by 'DOMAIN/xxxxx'
[51de4e9b-63c9-4a01-8a57-ea80c9caefdb] ERROR {org.apache.axis2.engine.AxisEngine} - Access Denied. org.apache.axis2.AxisFault: Access Denied.
        at org.wso2.carbon.server.admin.module.handler.AuthorizationHandler.doAuthorization(AuthorizationHandler.java:123)
        at org.wso2.carbon.server.admin.module.handler.AuthorizationHandler.invoke(AuthorizationHandler.java:87)
        at org.apache.axis2.engine.Phase.invokeHandler(Phase.java:340)
        at org.apache.axis2.engine.Phase.invoke(Phase.java:313)
        at org.apache.axis2.engine.AxisEngine.invoke(AxisEngine.java:261)
        at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:167)
        at org.apache.axis2.transport.http.util.RESTUtil.invokeAxisEngine(RESTUtil.java:144)
        at org.apache.axis2.transport.http.util.RESTUtil.processXMLRequest(RESTUtil.java:89)
        at org.apache.axis2.transport.http.AxisServlet$RestRequestProcessor.processXMLRequest(AxisServlet.java:840)
        at org.apache.axis2.transport.http.AxisServlet.doPost(AxisServlet.java:229)
        at org.wso2.carbon.core.transports.CarbonServlet.doPost(CarbonServlet.java:238)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
        at org.eclipse.equinox.http.servlet.internal.ServletRegistration.service(ServletRegistration.java:61)
        at org.eclipse.equinox.http.servlet.internal.ProxyServlet.processAlias(ProxyServlet.java:128)
        at org.eclipse.equinox.http.servlet.internal.ProxyServlet.service(ProxyServlet.java:68)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
        at org.wso2.carbon.tomcat.ext.servlet.DelegationServlet.service(DelegationServlet.java:68)
        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 org.owasp.csrfguard.CsrfGuardFilter.doFilter(CsrfGuardFilter.java:88)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at org.apache.catalina.filters.HttpHeaderSecurityFilter.doFilter(HttpHeaderSecurityFilter.java:126)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at org.wso2.carbon.tomcat.ext.filter.CharacterSetFilter.doFilter(CharacterSetFilter.java:65)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at org.apache.catalina.filters.HttpHeaderSecurityFilter.doFilter(HttpHeaderSecurityFilter.java:126)
        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:96)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
        at org.wso2.carbon.identity.context.rewrite.valve.TenantContextRewriteValve.invoke(TenantContextRewriteValve.java:107)
        at org.wso2.carbon.identity.cors.valve.CORSValve.invoke(CORSValve.java:98)
        at org.wso2.carbon.identity.authz.valve.AuthorizationValve.invoke(AuthorizationValve.java:110)
        at org.wso2.carbon.identity.auth.valve.AuthenticationValve.invoke(AuthenticationValve.java:102)
        at org.wso2.carbon.tomcat.ext.valves.CompositeValve.continueInvocation(CompositeValve.java:99)
        at org.wso2.carbon.tomcat.ext.valves.TomcatValveContainer.invokeValves(TomcatValveContainer.java:49)
        at org.wso2.carbon.tomcat.ext.valves.CompositeValve.invoke(CompositeValve.java:62)
        at org.wso2.carbon.tomcat.ext.valves.CarbonStuckThreadDetectionValve.invoke(CarbonStuckThreadDetectionValve.java:145)
        at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:690)
        at org.wso2.carbon.tomcat.ext.valves.CarbonContextCreatorValve.invoke(CarbonContextCreatorValve.java:57)
        at org.wso2.carbon.tomcat.ext.valves.RequestEncodingValve.invoke(RequestEncodingValve.java:49)
        at org.wso2.carbon.tomcat.ext.valves.RequestCorrelationIdValve.invoke(RequestCorrelationIdValve.java:126)
        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:373)
        at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
        at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
        at org.apache.tomcat.util.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590)
        at org.apache.tomcat.util.SocketProcessorBase.run(SocketProcessorBase.java:49)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
        at java.base/java.util.concurrent.ThreadPoolExecutor$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)

I checked the SOAP service separately using SOAP UI and identified that it works fine. A direction to resolve this issue is highly appreciated. Thanks in advance.