XML Application not working, Log says USER.ERROR



I have a strange behaviour on all my GXP2170 phones.
When I start an XML Application via a Softkey and the Phone is on the same subnet as the Server that is serving the XML, all is fine… but if I put the same phone in another subnet, having a router with partmapping in between, the phone does not render the XML Application. Instead the Log says:

Dec 19 16:31:16 GXP2170_GUI : USER.ERROR [00:0b:82:dc:77:0e][][2919232608] DownloadManager::DownloadThread:212->DownloadThread finished
Dec 19 16:31:16 GXP2170_GUI : USER.ERROR [00:0b:82:dc:77:0e][][3035271712] XMLAppModule::XMLAppController:126->Download “/var/user/xml_start.xml” failed with code: 56

Using a Browser and downloading the XML-Application I can see, that the file is transmitted correctly, so the router in between does not mangle anything.
I am also wondering about the Logentry saying “Download of xml_start.xml failed”… this line does not occur in the log, when the phone is in the same subnet and the XML-Application works fine…
Can someone please explain what this xml_start.xml file is and where the phone tries to retrieve it and what its content should be? What does the fail-code 56 mean?

Thank you