What is this Avast stubbornness that you speak of ?
I didn't want to annoy anyone with this criticism in this forum, sorry 
By stubbornness I meant, among other things, that all attempts to repair the invalid definitions date by clicking "check for updates" failed. That "repair app" in "setting"/"troubleshooting" was also unsuccessful. That normal un-installation and re-installation didn't change anything. At last the un-installation, then avastclear.exe, then the removal of all of Avast's folders and files and then the following re-installation was successful.
And even this new installation did not work with the offline-setup: trying to open the surface of the newly installed Avast only resulted in an error message. Only the online setup with "avast_free_antivirus_setup.exe" (file date 2016-08-19) then led to the goal.
And that Avast-FAV does not take over my saved settings even after several attempts - I have to enter it again manually.
I don't want to write about previous minor problems now, there were, but they could be solved more quickly.
All of this cost me several hours. The cause of these errors is not with me, because I had not changed the slightest thing about my computer constellation before the errors occurred, except for the automatic update from Avast 19 to 20
Isn't it understandable that I am angry that a piece of software caused me a lot of trouble?
But it is of course in no way directed against the members of this forum.Those that have responded (promptly) in this topic are Avast users just like yourself.
Have you tried to contacted Avast by another method (support ticket, etc.) ?
I know that this is a forum of Avast users - a good and helpful one. And I thank everyone who wanted to help me with their answers. I am sorry if in my annoyance I might have formulated the criticism of Avast Free Antivirus v20 in such a way that someone in the forum had to feel addressed. Sorry for that - it wasn't meant to be. It should only be a criticism of this antivirus software. But it is true, these passages in which I expressed my anger have do do nothing here, in a forum of users - so I will remove this from my posts.
I didn't contact Avast because I think a forum is faster.
Sorry again and meanwhile my Avast FAV is working so far.