Problem solved with the help or the very efficient DELL support crew. I had to set my BIOS to default and launch again the Ubuntu recovery.
Dear Dell/Linux users, I have been using for a little more than one year a Dell XPS 13 with Ubuntu bionic beaver initially installed. After a last upgrade to Ubuntu Hirtute Hippo, I had several concerns with bluetooth but also with different programs. Therfore I decided to use the DELL Recovery process to go back to the initial setup. It seems to work except, that right now, I am not root and I do not know what to do. There is a "Prepare for shipping to end user" icon, but when I launched it, I  got an identification window t appeared. The user is OEM and I have no idea of the password associated to the OEM account (it is not mine and I was never asked to choose any password for OEM).  Any hint or idea to help me would be very appreciated. Best,
Dell recovery problem