403/500 error when posting comment when user isn’t logged in

I have installed www.ads-software.com version 2.7.1.
When I try to post a comment without being logged in, I get the 403 server error message i.e
” 403 – Forbidden: Access is denied.
You do not have permission to view this directory or page using the credentials that you supplied”
or the 500 server error message i.e
” 500 – Internal server error.
There is a problem with the resource you are looking for, and it cannot be displayed”.
And the strange thing is I don’t get it the first time I post the comment without being logged in but I get it from the second time.

I think this is a server authentication issue – I tried several things to fix this.
1) My wordpress is installed on godaddy and i enabled anonymous authentication but that didn’t work.
2) I tried the .htaccess file – which seemed to have resolved 403 errors during log -in but in vain.
3) I tried to fix it at the code level but couldn’t do it.

I would be grateful to anyone who could help me with this.
Thank you.