When you use the "Trigger release in TFS/Team Services" post build action, if you specify the Collection Url without "DefaultCollection", you get an obscure error.  For example, if you specify this url "https://visualstudiocat.visualstudio.com/" instead of "https://visualstudiocat.visualstudio.com/DefaultCollection".

I was eventually able to figure out that I needed DefaultCollection after reading the [ReadMe |https://github.com/jenkinsci/tfs-plugin] for the plugin - but, this wasn't clear up front since there are places where you specify the url that you need to omit the DefaultCollection from the path and other places that you do need to include it.

This error message should be clearer, or...the plugin should automatically add it to the url if it's missing.  

Here's the error you get when running the job:

ERROR: Build step failed with exception
java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 3 column 1 path $
        at com.google.gson.stream.JsonReader.beginObject(JsonReader.java:387)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:189)
Caused: com.google.gson.JsonSyntaxException
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:200)
        at com.google.gson.Gson.fromJson(Gson.java:810)
        at com.google.gson.Gson.fromJson(Gson.java:775)
        at com.google.gson.Gson.fromJson(Gson.java:724)
        at com.google.gson.Gson.fromJson(Gson.java:696)
        at hudson.plugins.tfs.rm.ReleaseManagementHttpClient.GetReleaseDefinitions(ReleaseManagementHttpClient.java:43)
        at hudson.plugins.tfs.rm.ReleaseManagementCI.perform(ReleaseManagementCI.java:88)
        at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
        at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:736)
        at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:682)
        at hudson.maven.MavenModuleSetBuild$MavenModuleSetBuildExecution.post2(MavenModuleSetBuild.java:1073)
        at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:627)
        at hudson.model.Run.execute(Run.java:1762)
        at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:543)
        at hudson.model.ResourceController.execute(ResourceController.java:97)
        at hudson.model.Executor.run(Executor.java:421)