TestLink plugin can not verify developer key.
Here is the trace from the Jenkins console.
After this trace, please find a network capture of the XML RPC call (made with tcpdump)

Jenkins console:
----------------

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

FATAL: Error verifying developer key: Failed to parse server's response: The processing instruction target matching "[xX][mM][lL]" is not allowed.
br.eti.kinoshita.testlinkjavaapi.TestLinkAPIException: Error verifying developer key: Failed to parse server's response: The processing instruction target matching "[xX][mM][lL]" is not allowed.
at br.eti.kinoshita.testlinkjavaapi.MiscService.checkDevKey(MiscService.java:71)
at br.eti.kinoshita.testlinkjavaapi.TestLinkAPI.<init>(TestLinkAPI.java:150)
at hudson.plugins.testlink.TestLinkHandler.<init>(TestLinkHandler.java:99)
at hudson.plugins.testlink.TestLinkBuilder.createTestLinkHandler(TestLinkBuilder.java:320)
at hudson.plugins.testlink.TestLinkBuilder.perform(TestLinkBuilder.java:139)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:682)
at hudson.model.Build$RunnerImpl.build(Build.java:178)
at hudson.model.Build$RunnerImpl.doRun(Build.java:139)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:448)
at hudson.model.Run.run(Run.java:1376)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:230)
Caused by: org.apache.xmlrpc.client.XmlRpcClientException: Failed to parse server's response: The processing instruction target matching "[xX][mM][lL]" is not allowed.
at org.apache.xmlrpc.client.XmlRpcStreamTransport.readResponse(XmlRpcStreamTransport.java:188)
at org.apache.xmlrpc.client.XmlRpcStreamTransport.sendRequest(XmlRpcStreamTransport.java:156)
at org.apache.xmlrpc.client.XmlRpcHttpTransport.sendRequest(XmlRpcHttpTransport.java:143)
at org.apache.xmlrpc.client.XmlRpcSunHttpTransport.sendRequest(XmlRpcSunHttpTransport.java:69)
at org.apache.xmlrpc.client.XmlRpcClientWorker.execute(XmlRpcClientWorker.java:56)
at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:167)
at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:158)
at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:147)
at br.eti.kinoshita.testlinkjavaapi.BaseService.executeXmlRpcCall(BaseService.java:90)
at br.eti.kinoshita.testlinkjavaapi.MiscService.checkDevKey(MiscService.java:65)
... 13 more
Caused by: org.xml.sax.SAXParseException; lineNumber: 2; columnNumber: 6; The processing instruction target matching "[xX][mM][lL]" is not allowed.
at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
at org.apache.xerces.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
at org.apache.xmlrpc.client.XmlRpcStreamTransport.readResponse(XmlRpcStreamTransport.java:186)
... 22 more

XML RPC call capture:
--------------------

POST /lib/api/xmlrpc.php HTTP/1.1

Content-Type: text/xml

User-Agent: Apache XML RPC 3.1.3 (Sun HTTP Transport)

Cache-Control: no-cache

Pragma: no-cache

Host: testlink.neomura.int

Accept: text/html, image/gif, image/jpeg, ; q=.2, */; q=.2

Connection: keep-alive

Content-Length: 310

<?xml version="1.0" encoding="UTF-8"?><methodCall xmlns:ex="http://ws.apache.org/xmlrpc/namespaces/extensions"><methodName>tl.checkDevKey</methodName><params><param><value><struct><member><name>devKey</name><value>84d19df078791f4ed09ee99be327437d</value></member></struct></value></param></params></methodCall>HTTP/1.1 200 OK

Date: Thu, 01 Sep 2011 14:56:12 GMT

Server: Apache/2.2.14 (Ubuntu)

X-Powered-By: PHP/5.3.2-1ubuntu4.9

Connection: close

Content-Length: 164

Vary: Accept-Encoding

Content-Type: text/xml

<?xml version="1.0"?>
<methodResponse>
<params>
<param>
<value>
<boolean>1</boolean>
</value>
</param>
</params>
</methodResponse>