red tool eclipse plugin

Question

I try to support robotframework 5.1 to red tool. Whats mean this error:

org.rf.ide.core.environment.IRuntimeEnvironment$RuntimeEnvironmentException: Unable to communicate with XML-RPC server
    at org.rf.ide.core.environment.RobotCommandRpcExecutor.getGlobalVariables(RobotCommandRpcExecutor.java:156)
    at org.rf.ide.core.environment.RobotRuntimeEnvironment.getGlobalVariables(RobotRuntimeEnvironment.java:165)
    at org.rf.ide.core.testdata.model.RobotProjectHolder.configure(RobotProjectHolder.java:50)
    at org.robotframework.ide.eclipse.main.plugin.model.RobotProject.getRobotProjectHolder(RobotProject.java:88)
    at org.robotframework.ide.eclipse.main.plugin.project.build.libs.LibrariesBuilder.buildLibraries(LibrariesBuilder.java:212)
    at org.robotframework.ide.eclipse.main.plugin.project.build.RobotArtifactsBuilder.buildArtifacts(RobotArtifactsBuilder.java:109)
    at org.robotframework.ide.eclipse.main.plugin.project.build.RobotArtifactsBuilder.access$2(RobotArtifactsBuilder.java:82)
    at org.robotframework.ide.eclipse.main.plugin.project.build.RobotArtifactsBuilder$1.run(RobotArtifactsBuilder.java:49)
    at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)
Caused by: org.apache.xmlrpc.XmlRpcException: <class 'TypeError'>:cannot marshal <class 'robot.utils.dotdict.DotDict'> objects
    at org.apache.xmlrpc.client.XmlRpcStreamTransport.readResponse(XmlRpcStreamTransport.java:197)
    at org.apache.xmlrpc.client.XmlRpcStreamTransport.sendRequest(XmlRpcStreamTransport.java:156)
    at org.apache.xmlrpc.client.XmlRpcHttpTransport.sendRequest(XmlRpcHttpTransport.java:143)
    at org.apache.xmlrpc.client.XmlRpcSunHttpTransport.sendRequest(XmlRpcSunHttpTransport.java:69)
    at org.apache.xmlrpc.client.XmlRpcClientWorker.execute(XmlRpcClientWorker.java:56)
    at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:167)
    at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:137)
    at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:126)