2) I'm using 'silent-mode' of Avast constantly. In fact, i never tried to set it to regular-mode to see, if it stops logging all 3 seconds.
Did you already tried this, or, are you using Avast in regular-mode and had the problem nevertheless?
I always use Silent mode, and have no intention to try and use any other mode. Sorry.
Maybe explaining it might show me a way to find the problem on my system.
Just get to your Windows Firewall settings (you can search on google how to do it) and check both incoming and outcoming Rules there.
Search for anything Avast related (with red icons which signify the blocking being enabled). If you find blocked Avast related things, see if it's blocking something for the file named "AvastUI.exe" and disable that Rule in the Firewall.