Drupal\Core\File\Exception\FileException: Failed to unlink file 'public://ipless//ipless-ipless.watching--ipless.watching.css'. in Drupal\Core\File\FileSystem->delete() (line 327 of /var/www/html/web/core/lib/Drupal/Core/File/FileSystem.php).

You have this error because your files directory is not writable.
As I say in your previous ticket, you have to checks your public:// directory permission.

It looks like Drupal changes permissions for js and css folders after an update, see #3369960-7: type error on drush cr → for more.

@ressa, thanks for your redirection. Could it be a core issue?

@Damien Laguerre, sorry for my late reply, but, unfortunately it repeats every time I do a cache rebuild.

You're welcome @Orkut Murat Yılmaz. It looks like a Drupal core issue to me, without being sure ... In the documentation, for all folders, the owners are set to webuser:www-data, but it looks like Drupal changes the owner of js and css to www-data:www-data, and then the server is locked out, and can't access the content.

@ressa, I've changed my nginx configuration, in order to understand the root of the problem. I guess that I should open a core issue, if it doesn't exist already. Thank you again.

Thank you @Orkut Murat Yılmaz.