try to login, redirect to install page

Hi all,
my website homepage works but if I click on a link WordPress redirect to mysite/wp-admin/install.php.
I tried to login to mysite/wp-login.php and it also redirects to mysite/wp-admin/install.php

The install procedure asks for language, site title and email but says Username: User(s) already exists.
Site recently updeted to 4.8.3. Alredy cleared browser cache and cookies. What happened?
Thanks in advance.