Hi,
Thank you for all your answers.
Here are my comments:
1. Pondus: I tried the repair before posting my question. It didn't help.
2. DavidR: I never installed a firewall, but I see that the Windows firewall is up and running. I disabled it, but this does not seem to change anything here. I did not install any other anti-virus software.
3. stibi: yes, this configuration worked in the past. I did not intentionally change anything related to this matter.
4. Luukjr: I don't find any Secure DNS option in the Settings of the Avast user interface (probably not part of the free version).
This last answer gave me the idea to disable all options in the Setting and then to turn them on one at a time. Now that I re-activated all of them, I can still use my browsers to access the Internet. So the problem is gone as strangely as it arrived...
Hoping that this will still work tomorrow morning.
Thanks again for your help.
Jean-Luc