The same problem is reproducible on Jenkins ver. 1.499, TestLink 1.9.5 and Testlink plugin 3.1.8
Preparing TestLink client API.
Using TestLink URL: http://****/testlink/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.util.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:66)
at br.eti.kinoshita.testlinkjavaapi.TestLinkAPI.<init>(TestLinkAPI.java:162)
at hudson.plugins.testlink.TestLinkBuilder.getTestLinkSite(TestLinkBuilder.java:244)
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:810)
at hudson.model.Build$BuildExecution.build(Build.java:199)
at hudson.model.Build$BuildExecution.doRun(Build.java:160)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:592)
at hudson.model.Run.execute(Run.java:1543)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:236)
Caused by: org.apache.xmlrpc.client.XmlRpcClientException: Failed to parse server's response: The processing instruction target matching "[xX][mM][lL]" is not allowed.
at org.apache.xmlrpc.client.XmlRpcStreamTransport.readResponse(XmlRpcStreamTransport.java:188)
at org.apache.xmlrpc.client.XmlRpcStreamTransport.sendRequest(XmlRpcStreamTransport.java:156)
at org.apache.xmlrpc.client.XmlRpcHttpTransport.sendRequest(XmlRpcHttpTransport.java:143)
at org.apache.xmlrpc.client.XmlRpcSunHttpTransport.sendRequest(XmlRpcSunHttpTransport.java:69)
at org.apache.xmlrpc.client.XmlRpcClientWorker.execute(XmlRpcClientWorker.java:56)
at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:167)
at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:158)
at org.apache.xmlrpc.client.XmlRpcClient.execute(XmlRpcClient.java:147)
at br.eti.kinoshita.testlinkjavaapi.BaseService.executeXmlRpcCall(BaseService.java:90)
at br.eti.kinoshita.testlinkjavaapi.MiscService.checkDevKey(MiscService.java:62)
... 12 more
Caused by: org.xml.sax.SAXParseException; lineNumber: 2; columnNumber: 6; The processing instruction target matching "[xX][mM][lL]" is not allowed.
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1234)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:525)
at org.apache.xmlrpc.client.XmlRpcStreamTransport.readResponse(XmlRpcStreamTransport.java:186)