Problem solved
Thank you for helping me.
Understanding the mess I was swimming in, I solved the pb with the following steps.
- New installation & initialisation of an empty Moodle 3.7.1
- Modification of the config file to link it to the older bdd and the older moodledata folder
- Here moodle offered me to clean up the rest of the plugins I was dragging with that older bdd and upgraded the database nicely
- everything works fine for the admin accounts & the user accounts.
I can go further ! And I've got a better understanding of the architecture of moodle...