401.shtml unauthorized error after htaccess http to https redirection

Strangely i have experienced 401.shtml unauthorized error after i have enabled http to https redirection through htaccess.

These are the steps i have followed.,

• I have already configured htaccess password protection to my admin part of the website, it allows me to navigate to the admin part after password verification is successfull 

  ex: http://<domain name>/admin/
  
  

• I have enabled ssl on the website and all the urls redirected from http to https://www version through redirect rules in main htaccess file, which is present in public_html directory
• Now, when i enter into my admin part now, it shows 401.shtml unauthorized error. It just dragged hours of my time without know what has happening. Surprisingly when i enter the https version of the admin URL, it worked fine.
• After Googling i have found that adding following htaccess config to the htaccess present inside admin folder resolved the issue.
• I have configured following htaccess config in admin/.htaccess, it worked fine after that.