Problem with wp-admin

added ob_start();
in wp-config.php

What? Why? That’s absolutely the wrong thing to do!

Do proper debugging… Errors like this are logged. Check the error log on your server. If you can’t find the log, please contact your host.

Meantime, enable wp_debug and wp_debug_log and after an error, look at wp-content/debug.log to see if anything gets logged there. https://www.ads-software.com/support/article/debugging-in-wordpress/

You can also try this: Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.

If you cannot access wp-admin, there are other ways to deactivate plugins:? https://www.ads-software.com/support/article/faq-troubleshooting/#how-to-deactivate-all-plugins-when-not-able-to-access-the-administrative-menus

Agreed with @sterndata

Check WordPress RAW logs.
access.log
error.log

Maybe you will see the exact error causing the issue.