Hi,
i solved the problem by myself.
For the people which have the same problems...
It was the Portscanner from the Intrusion Prevention from the Anti-Virus Suite.
I could re-enact the situation with other systems and that's the reason why i could say it was definitely the Portscanner.
The crazy thing was that disabling the AV-Suite didn't show any effect.
Only after i set the settings in the AV-suite that the IPS shouldn't control the ports everthing works fine.
Hard stuff but now it works...
Cheers
Markus