Hello,
are the Avast log contents deleted after some time? At the moment I find only entries from this month.
Yes they are deleted after some time. Open up Avast GUI, click on settings. On general tab on the left hand side you will see maintenance tab down below, click on it, scroll down a bit, and you will see a heading called Auto-Cleanup for Avast logs.
Thank you.
I increased it a little. 
Your welcome