Made a VPN and changed to "hudson.plguins.mantis".
Got a lot more log messages now 
Here we go. Just opened one job config and got:
{{May 28, 2015 9:40:02 PM INFO hudson.plugins.mantis.soap.mantis120.MantisSessionImpl
Mantis version is 1.2.X
May 28, 2015 9:40:02 PM FINE hudson.plugins.mantis.soap.LogHandler
Request: <?xml version="1.0" encoding="UTF-8"?><soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soapenv:Body>
<ns1:mc_projects_get_user_accessible soapenv:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" xmlns:ns1="http://futureware.biz/mantisconnect">
<username xsi:type="xsd:string">xxx</username>
<password xsi:type="xsd:string">xxx</password>
</ns1:mc_projects_get_user_accessible>
</soapenv:Body>
</soapenv:Envelope>
May 28, 2015 9:40:02 PM FINE hudson.plugins.mantis.soap.LogHandler
Response: (null)
May 28, 2015 9:40:02 PM FINE hudson.plugins.mantis.soap.LogHandler
Request: <?xml version="1.0" encoding="UTF-8"?><soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soapenv:Body>
<ns1:mc_projects_get_user_accessible soapenv:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" xmlns:ns1="http://futureware.biz/mantisconnect">
<username xsi:type="xsd:string">xxx</username>
<password xsi:type="xsd:string">xxx</password>
</ns1:mc_projects_get_user_accessible>
</soapenv:Body>
</soapenv:Envelope>
May 28, 2015 9:40:02 PM FINE hudson.plugins.mantis.soap.LogHandler
Response: <?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ns1="http://futureware.biz/mantisconnect" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"><SOAP-ENV:Body><ns1:mc_projects_get_user_accessibleResponse><return SOAP-ENC:arrayType="ns1:ProjectData[16]" xsi:type="SOAP-ENC:Array"><item xsi:type="ns1:ProjectData"><id xsi:type="xsd:integer">8</id><name xsi:type="xsd:string">xxx</name><status xsi:type="ns1:ObjectRef"><id xsi:type="xsd:integer">50</id><name xsi:type="xsd:string">stabil</name></status><enabled xsi:type="xsd:boolean">true</enabled><view_state xsi:type="ns1:ObjectRef"><id xsi:type="xsd:integer">10</id><name xsi:type="xsd:string">öffentlich</name></view_state><access_min xsi:type="ns1:ObjectRef"><id xsi:type="xsd:integer">10</id><name xsi:type="xsd:string">Betrachter</name></access_min><file_path xsi:type="xsd:string"></file_path>
... Big, big, project list xml ...
<subprojects SOAP-ENC:arrayType="ns1:ProjectData[0]" xsi:type="SOAP-ENC:Array"/></item></return></ns1:mc_projects_get_user_accessibleResponse></SOAP-ENV:Body></SOAP-ENV:Envelope>
}}
Looks like connection is ok and list of projects and categories has been received.
Could this be an issue of latest java 8 u45 update? That's all I remember doing the last weeks.
Any ideas?
Just upgraded LTS version to Jenkins ver. 1.596.3 but no success.
There is only one log entry:
Failed to connect http://x.x.x.x/mantis/ (cause: org.xml.sax.SAXParseException: Content is not allowed in prolog.).