UPDATE:today night I was planning to reinstall the whole mobile phone because I also tried “easy-tools” to recover files like recuva or AndroidApps (DiscDigger) that promise a restoration of internal file (all of these apps were not able to find any of the existing files.
but it can still be worse… I now also noticed that the automatic “Google Drive Backup” within WhatsApp overwrited the existing full backup - hence I’m afraid that I have also lost all pictures that has been shared and received within WhatsApp. - If this is the case and you run into the same issues:
please go sure to instantly freeze the existing google drive backup (if enabled) by disabling the WLAN and mobile-data connection!