Thanks again for your suggestions so far. I will give it a trial.
Hi Dot 07,
Many thanks for your suggestion. In fact, I accessed the plugin directory of my website and deleted the plugin Users Ultra (I did it before your suggestion) and tried again to log in to my site. Still WordPress did not allow me to access the site backend.
I even tried to request a new password but wordpress could not recognize anymore the email address that I used to create the website. I am having this issue only with the website where I was installing UU plugin. My other wordpress websites installed on the same host are working without any issue.
May you have any idea of what I can try, please do not hesitate to let me know.
Thanks again for your reply.