@Armelle, sorry to hear that you have faced this issue.
Have you enabled the User Synchronization option and enabled password and username?
Also, can you please share a video of the issue so we can reproduce that on your end and assist you further with this.
It would be helpful, if you can share a temporary admin access of WordPress and Moodle, along with FTP, so we can check and fix this on your end.
While sharing the access, don’t forget to mark the response as private.