Check the terminal, where namecoinj is running while the curl command is executed. There might be an exception, including "HTTP response code: 500" (see below), which in my case correlates with the curl command. I think, there lies the dog buried.
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_162]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_162]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_162]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_162]
at com.googlecode.jsonrpc4j.JsonRpcServer.invoke(JsonRpcServer.java:513) [jsonrpc4j-1.1.jar!/:na]
at com.googlecode.jsonrpc4j.JsonRpcServer.handleObject(JsonRpcServer.java:384) [jsonrpc4j-1.1.jar!/:na]
at com.googlecode.jsonrpc4j.JsonRpcServer.handleNode(JsonRpcServer.java:293) [jsonrpc4j-1.1.jar!/:na]
at com.googlecode.jsonrpc4j.JsonRpcServer.handle(JsonRpcServer.java:230) [jsonrpc4j-1.1.jar!/:na]
at com.googlecode.jsonrpc4j.JsonRpcServer.handle(JsonRpcServer.java:207) [jsonrpc4j-1.1.jar!/:na]
at com.googlecode.jsonrpc4j.spring.JsonServiceExporter.handleRequest(JsonServiceExporter.java:40) [jsonrpc4j-1.1.jar!/:na]
at org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter.handle(HttpRequestHandlerAdapter.java:51) [spring-webmvc-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967) [spring-webmvc-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901) [spring-webmvc-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) [spring-webmvc-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872) [spring-webmvc-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:661) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) [spring-webmvc-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-embed-websocket-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:105) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-4.3.9.RELEASE.jar!/:4.3.9.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:799) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:861) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_162]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_162]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.15.jar!/:8.5.15]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_162]
Caused by: java.io.IOException: Server returned HTTP response code: 500 for URL: https://namecoin.webbtc.com/name/d/domob.json?history&with_height&with_rawtx&with_mrkl_branch&with_tx_idx&raw
at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1894) ~[na:1.8.0_162]
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1492) ~[na:1.8.0_162]
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:263) ~[na:1.8.0_162]
at java.net.URL.openStream(URL.java:1045) ~[na:1.8.0_162]
at com.fasterxml.jackson.core.JsonFactory._optimizedStreamFromURL(JsonFactory.java:1547) ~[jackson-core-2.8.8.jar!/:2.8.8]
at com.fasterxml.jackson.core.JsonFactory.createParser(JsonFactory.java:783) ~[jackson-core-2.8.8.jar!/:2.8.8]
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2797) ~[jackson-databind-2.8.8.jar!/:2.8.8]
at org.libdohj.names.NameLookupLatestRestMerkleApi.getUntrustedNameHistory(NameLookupLatestRestMerkleApi.java:130) ~[libdohj-namecoin-0.14-SNAPSHOT.jar!/:na]
at org.libdohj.names.NameLookupLatestRestMerkleApi.getLatestUntrustedNameData(NameLookupLatestRestMerkleApi.java:103) ~[libdohj-namecoin-0.14-SNAPSHOT.jar!/:na]
at org.libdohj.names.NameLookupLatestRestMerkleApi.getNameTransaction(NameLookupLatestRestMerkleApi.java:63) ~[libdohj-namecoin-0.14-SNAPSHOT.jar!/:na]
at org.namecoin.bitcoinj.spring.service.NameLookupService.name_show(NameLookupService.java:157) ~[bitcoinj-server-0.2.7-SNAPSHOT.jar!/:na]
... 55 common frames omitted
I did not set the environment variable as recommended in the instructions. And I used OpendJDK.