callCallBack not called although SIP status 180 (Ringing) is correctly returned (1.6.8)

[fs008]

I tried the first example in the docu (called "Setup") on a raspberry pi (debian 11, kernel 6.1, clean install)
SIP registration works flawless, but pyVoIP does not even call the callCallBack procedure when called by a SIP INVITE.
pyVoIP.DEBUG = True brings up this message:

Raw:
b'INVITE sip:[email protected]:5060;transport=UDP SIP/2.0\r\nVia: SIP/2.0/UDP 192.168.1.9;rport;branch=z9hG4bK5D59yS0rtFv5m\r\nMax-Forwards: 70\r\nFrom: "Comfortel3500" sip:[email protected]:5060;tag=7yF72QQc20D0j\r\nTo: sip:[email protected]:5060;transport=UDP\r\nCall-ID: d4993256-3ce1-123e-9cbe-dd8a4e9ce850\r\nCSeq: 34360 INVITE\r\nContact: sip:[email protected]:5060\r\nUser-Agent: Auerswald PBX sofia-sip/1.12.11\r\nAllow: INVITE, ACK, BYE, CANCEL, OPTIONS, PRACK, MESSAGE, REGISTER, SUBSCRIBE, INFO, UPDATE, REFER\r\nSupported: timer\r\nAllow-Events: auerswald-systel-config\r\nSession-Expires: 1800\r\nMin-SE: 90\r\nContent-Length: 0\r\n\r\n'
New call!
Error on header parsing: 'c'

So pyVoIP recognizes the call (and answers with a SIP status180), but why doesn't it call the callCallBack procedure ??? What is the error on header parsing ?