Core Error in Server Error Log?

See some solutions/tips at https://stackoverflow.com/questions/15850845/request-exceeded-the-limit-of-10-internal-redirects-due-to-probable-configuratio

Thanks. Sorry for the delayed response.

I am also seeing the following error, related to the following code in functions.

mod_fcgid: stderr: PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘extended_editor_mce_buttons_2’ not found or invalid function name in /home/user/domain.net/wp-includes/class-wp-hook.php on line 289, referer: https://www.domain.net/about/

// Disable All TinyMCE Buttons
if (isset($wp_version)) {
    add_filter('mce_buttons', 'extended_editor_mce_buttons', 0);
    add_filter('mce_buttons_2', 'extended_editor_mce_buttons_2', 0);
}

// TinyMCE: First line toolbar customizations
if( !function_exists('base_extended_editor_mce_buttons') ){
    function base_extended_editor_mce_buttons($buttons) {
        return array( 'formatselect', 'template', 'bold', 'italic', 'link', 'wp_adv' );
    }
    add_filter("mce_buttons", "base_extended_editor_mce_buttons", 0);
}

// TinyMCE: Second line toolbar customizations
if( !function_exists('base_extended_editor_mce_buttons_2') ){
    function base_extended_editor_mce_buttons_2($buttons) {
        return array( 'pastetext', 'charmap', 'undo', 'redo', 'code' );
    }
    add_filter("mce_buttons_2", "base_extended_editor_mce_buttons_2", 0);
}

This is running on WP 5.4.9, PHP 7.4. Are these connected or two separate errors?

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).

Thanks, I will try this.