OK thank you falks for your help.
It’s an old install and I had no other theme/component install. So you couldn’t change my theme or disable all the component.
So I restored an old backup, updated my theme, then updated Discourse. It’s better, but I still have one error (out of 2 before).
Here is the error:
Uncaught ReferenceError: Discourse is not defined
<anonymous> /assets/start-discourse-efa4e5abfbd1b50b5152ffbe64d5dcea9f7c33f766dcc6387e2711f0f2112148.js:1
Here is what contains this file:
Discourse.start();
Do I have an old version of the default theme? (the current theme was created in 2016 by just adding some CSS and JS to the default theme)
PS : The website is running again because I’m using the backup without updating Discourse.