Thje problem is that my Avast Internet Security is trying to find updates of virus signatures and is permamently in the infinite search of this update.
The last update of virus signatures was on May 21st and the programm shows error message that my virus signatures need to be updated.
Repair and reboot https://support.avast.com/en-eu/article/Repair-Antivirus
If Repair doesn’t fix the problem, try the following:
If your Avast settings are not the default settings and you want them restored after the Clean Install, follow this before removing Avast.
https://goo.gl/TXjM7h
Clean Install of Avast:
https://goo.gl/4Ptzkf
If you need additional help with the Clean Install, watch this:
https://youtu.be/p-h3myRD51A