I'm running into the same issue with a local install of Jenkins and TestLink. Configuration looks correct on my side:

Preparing TestLink client API.
Using TestLink URL: http://localhost/testlink/lib/api/xmlrpc.php

FATAL: java.util.HashMap cannot be cast to [Ljava.lang.Object;
java.lang.ClassCastException: java.util.HashMap cannot be cast to [Ljava.lang.Object;
at br.eti.kinoshita.testlinkjavaapi.util.Util.castToArray(Util.java:642)
at br.eti.kinoshita.testlinkjavaapi.TestProjectService.getTestProjectByName(TestProjectService.java:111)
at br.eti.kinoshita.testlinkjavaapi.TestLinkAPI.getTestProjectByName(TestLinkAPI.java:485)
at hudson.plugins.testlink.TestLinkBuilder.getTestLinkSite(TestLinkBuilder.java:246)
at hudson.plugins.testlink.TestLinkBuilder.perform(TestLinkBuilder.java:134)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:804)
at hudson.model.Build$BuildExecution.build(Build.java:199)
at hudson.model.Build$BuildExecution.doRun(Build.java:160)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:586)
at hudson.model.Run.execute(Run.java:1516)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:236)

Hello @Supratim,

Sorry for the delay. I've been busy with other project/plug-ins and couldn't find a cycle to work on Jenkins, TestLink or on this plug-in. Sorry.

Since last Wednesday I've been working on these tools, but first I had to release a 1.9.4 testlink-java-api version. Yesterday I released another version of the testlink-java-api, 1.9.5-0. Now I'm working on these issues for 1.9.5/1.9.6.

Thanks for confirming this issue @Fred. I'll take a look on this. Looks like a simple mismatch between the parameters of the testlink-java-api used by the plug-in, and TestLink.

BTW, let me say that today I'm extremely happy, to receive a comment in this plug-in by you. When I wrote this plug-in, the initial version, was highly influenced by your blog entries. I was working on a telecom company, as QA engineer for the first time, and faced the need to integrate my CI jobs with the Test Management team. Not knowing much about test management, your blog was one of the most important resources. So, thank you very much!!!

Bug confirmed when using the plug-in with TestLlink 1.9.6. Apparently, TestLink XML-RPC method getTestProjectByName, which used to return an array, is now returning a map.

Please see [1] to track the changes, but probably tomorrow (Saturday) it will be fixed in trunk, and a release is to be cut in the next days.

Thanks!

[1] https://github.com/kinow/testlink-java-api/issues/12

Code changed in jenkins
User: Bruno P. Kinoshita
Path:
pom.xml
http://jenkins-ci.org/commit/testlink-plugin/b75daa2edbfdaf4cac83c90bf003f940bf4d917a
Log:
[FIXED JENKINS-17442] updated testlink-java-api to 1.9.6-0, which fixes the communication issue