Hi Harald,
Thanks for writing in!
This is a situation we've seen a few times before - and actually have a knowledge base article that goes over some of the possibilities here:
http://www.pingman.com/kb/37If none of the points in the above article seem to address your situation - would you possibly be able to send us a support ticket directly from PingPlotter ("Help" -> "Email PingPlotter Support")? This will help give us a look at your current configuration - which may help us provide some additional insight on this issue.
Please let us know if you have any questions!
Best wishes,
-Gary