I found a file called install.php at /wp-content/ directory which contained the line causing the issue:
defined( 'WTWP__INSTALL_PLUGIN_PATH' ) or define( 'WTWP__INSTALL_PLUGIN_PATH', 'welcome-to-wordpress/welcome-to-wordpress.php' );
Upon deleting the file, I was able to resolve the issue.
This is most probably a file that was placed by a hosting provider by default long with a plugin that has been later deleted, causing the error to popup.