404 error after updgrading WordPress to 5.9.1

Hi,

I updated the WordPress version of the site last night and it was successful but after that, I can no longer access the wp-admin or the dashboard of the site.

I have done troubleshooting such as:

* renaming the plugins folder and then going back to the wp admin (or the customized URL since we are using a custom URL to hide the login URL) but the problem still persists
* second I have downloaded a new WordPress zip file and uploaded the upgrade.php and version.php file to the server using FTP to replace the current upgrade.php file on the site but these still don’t work.
* renaming the theme’s folder URL but still doesn’t work.
* enable wp debugging on the site and I was getting the notice: failed to send buffer of zlib output compression (0)……… so I have added this code to the function.php of my current theme remove_action( 'shutdown', 'wp_ob_end_flush_all', 1 );

I have done all these but still I am unable to access the admin dashboard. Any other possible fix for this issue? I have checked forums and I couldn’t find any other solution.