Hi Tharuna,
To start with the cause of the problem, it was the .htaccess.
We use this file as htaccess file :
https://github.com/joomla/joomla-websites/blob/master/configuration-baselines/.htaccess-joomla
And we had this line inserted :
RewriteRule ^administrator\/components\/com_csvi\/rantai\/rantai\.php$ - [L]
But had to modify the position within the file (now on line 457) to fix the error.
This site was in offline modus and for some reason gave a 503 error and that was confusing. After we put the site online the error changed to 404 and made it a lot easier to find the issue. (very often .htaccess

).
So it's fixed now.