1. Wrong File Permissions
The file permission for all files in our server should be set to 644 and all directories to 755.
You may see a 403 Error you have insecure file permissions like 666 and 777.
Magento Built-In Backup System is known to reset file permissions to 666 and 777.
If you have SSH Access, you can resolve this easily.
SSH into your account and switch to the public_html directory (VERY IMPORTANT):-
Then input the following command:-
find . -type d -print0 | xargs -0 chmod 0755 && find . -type f -print0 | xargs -0 chmod 0644
If you do not have SSH Access or you are afraid of setting things wrongly, you can run our script at GitHub in your web browser.
Your script or application may have triggered ModSecurity, our Web Application Firewall which is an extra layer of protection for your website against hackers and exploits.
You can temporarily disable ModSecurity in your cPanel --> ModSecurity to determine if this is the cause of the error.
3. Rules in .htaccess File
You may have some rules in your .htaccess file which blocks access to some files, directories or the whole website.