I have web application under the folder of SaiSun EWay Gateway but unfortunately after activation of Cloudflare on my domain now this application has giving error of Unauthorized access error 401 on the login screen even after giving correct login and password.
I see the login page just fine on my end. DNS propagation can take a hours and wondering how long after adding the site to Cloudflare did you wait before testing?
I also get the login page just fine, so I assume you meant to say you get the the 401 error after you try to log in, right?
That error assumingly comes from your end, so you’d need to check why your server does not accept valid credentials anymore. It is very unlikely that Cloudflare tampers with the provided data but that bit would be also covered if you check on your end why the data is not accepted.