!! SOLVED !!
Ok, the problem was the Buffalo SmartWeb switch! 
Some of the fields for DoS were enabled, and apparently that caused the 5060 traffic to be seen as a Denial of Service, and the switch was blocking the access.
I did not look at the switch first, because my softphone configured just fine. X-Lite got right onto the server no questions asked.
Thank you all for reading along...
Christian