Hi Nicolas and Ralf. Sorry I've been away for a couple of hours. I think I've solved my problems because I recover right now all the lost functions.
I'm not an expert, I cannot explain the reasons of the problems, I can only describe what I did. I replaced my original config file by the one Nicolas sent this morning and I deleted a config.php.save file which was not supposed to be there. Then, I canceled the "s" in the https string to return to http option. And I got access to my admin and users accounts. Of course, it doesn't work in https, I'll do first a backup and a new try in few days.
I confirm that I run the original 3.12.0 version with no plug-in and no change in any other file. And the message error was the one I wrote.
I don't know what you can conclude from this story, but thanks a lot for your interest and your help.