I seen a few topics with this unresolved. I had the same problem. It was the .htaccess file on your main domain that was the problem so i deleted the contents of it and my addon domain works fine. But i am not using my main domain anymore since i moved it to another host hence the reason why i just deleted the contents in the .htaccess file. I am sure the problem lies somewhere in the .htaccess file on the main domain. Good luck