I can’t login after uploading old .html files on WP .php site

Both can’t co-exist. The browser will always choose the index file that is first alphabetically, in this case index.htm, and WordPress relies on its index (index.php) being primary to function.

To fix this, you will either need to renamed index.html to something else, like oldsite.html, or add this to the top of your .htaccess file:

DirectoryIndex index.php index.html index.htm

Thank you very much, James.
I just renamed the file as you recommended but did not touch the .htaccess file because as you saw https://www.agopunturascientifica.it opening page shows in the upper part the HTML content of the (deleted) index.html file and in the lower that of index.php.
Anyway, at the moment, the biggest problem is that I cannot log in at all … ?? Only a white screen of death. Any suggestions?

A white screen usually means that PHP is crashing due to a function conflict introduced by a plugin or theme.

Try manually resetting your plugins (no Dashboard access required). If that resolves the issue, reactivate each one individually until you find the cause.

If that does not resolve the issue, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue (theme functions can interfere like plugins).

Dear James,

I tried your suggestions but nothing to do. This installation of WordPress is defunct. Thus, I will take advantage of this to let WordPress down definitely and come back to a simpler modern html site. No need of plugins, very rare crashes, and working offline. Thank you very much for your kindness.