It looks like sip.qsc.de does not support IP fragmentation. The initial INVITE without authentication information gets a 407 "proxy authentication" response. When the authentication information is provided in the next INVITE the message becomes too large, so the UDP datagram is fragmented by IP layer. Since there is no answer from qsc I suspect they do not support IP fragments.
If they don't support TCP, then they would certainly need to support IP fragmentation as they would not comply to any of the SIP RFCs otherwise.
It's quite odd if qsc does not support IP fragmentation. I don't know of any ITSP or SIP server not supporting IP fragmentation. I know some phones, but not servers.
Your ethereal capture shows quite clearly the problem is with QSC server not answering when the message is fragmented. If they do not support TCP nor IP fragments there is not much we can do about it except trying to reduce message size, but it is not a good solution since sip messages seem to grow as new features are added.