I have been trying to use Workflow to check out a secured Subversion repository.

My script is this:
def creds = com.cloudbees.plugins.credentials.CredentialsProvider.lookupCredentials(
com.cloudbees.plugins.credentials.common.StandardUsernameCredentials.class,
Jenkins.instance,
null,
null
);
def cid = creds[0].id
node {
checkout scm: [ $class: "SubversionSCM", locations: [ remote:'https://secure3.svnrepository.com/myrepo/trunk', credentialsId: cid] ]
}

I have only one credential stored in Jenkins, the one for the svn repo, so that's why I am grabbing it programatically.

The output is:
Running: Allocate node : Start
Running on master in /var/jenkins_home/workspace/WorkflowTest
Running: Allocate node : Body : Start
Running: General SCM
Running: Allocate node : Body : End
Running: Allocate node : End
Running: End of Workflow
java.lang.ClassCastException: sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl cannot be cast to java.lang.Class
at org.jenkinsci.plugins.workflow.structs.DescribableHelper.coerce(DescribableHelper.java:161)
at org.jenkinsci.plugins.workflow.structs.DescribableHelper.buildArguments(DescribableHelper.java:133)
at org.jenkinsci.plugins.workflow.structs.DescribableHelper.instantiate(DescribableHelper.java:81)
at org.jenkinsci.plugins.workflow.structs.DescribableHelper.coerce(DescribableHelper.java:183)
at org.jenkinsci.plugins.workflow.structs.DescribableHelper.buildArguments(DescribableHelper.java:133)
at org.jenkinsci.plugins.workflow.structs.DescribableHelper.instantiate(DescribableHelper.java:81)
at org.jenkinsci.plugins.workflow.steps.StepDescriptor.newInstance(StepDescriptor.java:95)
at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:133)
at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:98)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:45)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:42)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:108)
at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.methodCall(DefaultInvoker.java:15)
at WorkflowScript.run(WorkflowScript:9)
at Unknown.Unknown(Unknown)
at __cps.transform__(Native Method)
at com.cloudbees.groovy.cps.impl.ContinuationGroup.methodCall(ContinuationGroup.java:69)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.dispatchOrArg(FunctionCallBlock.java:100)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.fixArg(FunctionCallBlock.java:76)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.CollectionLiteralBlock$ContinuationImpl.dispatch(CollectionLiteralBlock.java:55)
at com.cloudbees.groovy.cps.impl.CollectionLiteralBlock$ContinuationImpl.item(CollectionLiteralBlock.java:45)
at sun.reflect.GeneratedMethodAccessor240.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.CollectionLiteralBlock$ContinuationImpl.dispatch(CollectionLiteralBlock.java:55)
at com.cloudbees.groovy.cps.impl.CollectionLiteralBlock$ContinuationImpl.item(CollectionLiteralBlock.java:45)
at sun.reflect.GeneratedMethodAccessor240.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.CollectionLiteralBlock$ContinuationImpl.dispatch(CollectionLiteralBlock.java:55)
at com.cloudbees.groovy.cps.impl.CollectionLiteralBlock$ContinuationImpl.item(CollectionLiteralBlock.java:45)
at sun.reflect.GeneratedMethodAccessor240.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.LocalVariableBlock$LocalVariable.get(LocalVariableBlock.java:33)
at com.cloudbees.groovy.cps.LValueBlock$GetAdapter.receive(LValueBlock.java:30)
at com.cloudbees.groovy.cps.impl.LocalVariableBlock.evalLValue(LocalVariableBlock.java:22)
at com.cloudbees.groovy.cps.LValueBlock$BlockImpl.eval(LValueBlock.java:55)
at com.cloudbees.groovy.cps.LValueBlock.eval(LValueBlock.java:16)
at com.cloudbees.groovy.cps.Next.step(Next.java:58)
at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:145)
at org.jenkinsci.plugins.workflow.cps.CpsThread.runNextChunk(CpsThread.java:164)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.run(CpsThreadGroup.java:267)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.access$000(CpsThreadGroup.java:70)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:176)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:174)
at org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService$2.call(CpsVmExecutorService.java:47)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:111)
at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)
Finished: FAILURE

I used some of the info on https://groups.google.com/forum/#!topic/jenkinsci-users/GCLYCl7q_fI but I still can't get it to work.