403 is a different error. Can you please check your error log? You can find it in DirectAdmin.
Typically, it means the filesystem permissions are wrong. Files should be 0644 (owner read/write access, everyone else read only) and folders should be 0755 (owner read/write and enter directory, everyone else read and enter only).
Another problem is that you're trying to view a directory in your browser, but no index file exists, and the indexes option is not turned on for that directory. So if you go to mysite.example/blog/, inside of your account, there must be a folder ~/domains/mysite.example/blog/ and inside the folder must be index.html, index.htm, index.php, etc. Or, if you want people to see what is inside the folder, which is not usually a good thing, then you need to add a line to the file named .htaccess in that folder:
Otherwise, the error log may have some other reasoning this is the case.