You can use the steps below to help diagnose whether an error is related to WordPress core files.

Prepare your WordPress site for the troubleshooting:

Note: You should always make a backup of your site before performing any troubleshooting.

Disable any caching plugins.

Disable WP_CACHE if your site uses it.

Look for a core file-related error:

Enable WordPress debug mode.

You can enable WP_DEBUG and WP_DEBUG_LOG in the wp-config.php file. The contents of the debug.log file can help you and your site administrator narrow down the problem to a specific plugin, theme, or configuration option that might be causing a problem. For assistance, see: https://wordpress.org/support/article/debugging-in-wordpress/

Visit your site and check to see if there is a PHP error.

If there is a PHP error, check to see if a WordPress core file or folder is referenced in the file path.

Note: Often these errors will reference a file in the wp-admin or wp-includes folders.

Below is an example:

        Fatal error: require(): Failed opening required '/home/user/public_html/wp-includes/class-wp.php'

If the file path in the error contains a reference to a core file or folder, then you can move on to fixing the issue. Check article: https://help.tsohost.com/knowledge-base/article/12092