1) You may be using an ".htaccess" file, which may have an incorrect command inside it. Try clearing the content of ".htaccess" file to see if it resolves your issue.
2) The file that you are trying to browse is having a WRITE-permission to the GROUP. Try removing the write-permission for the group by using a permission like 755. You can change file permissions thru your FTP Client or your CPanel File Manager.
If the above tricks doesn't resolve your problem, then please open a support ticket and we'll take care of this issue.