Add to cart gives me a 403 Forbidden error?!

Hi All and thanks in advance for any help or guidance offered!
Ive recently installed the latest Ceon protx direct module and enabled 3D secure checks. - Only thing is now that when a customer or myself login to our store, and try and add a product to their cart the page diverts to a '403 Forbidden' Page and displays:

You don't have permission to access /index.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Im unsure of why this has happened as I havent modified/overide any of the index files? Can anyone point me in the right direction of where to start please?

many Thanks

You were not getting this error before the mod add-on?

Check the mod files and make sure you have included all the files.

You may also want to run the Debug utility found in the downloads section.

Tina

Hi Tina
Many thanks for your reply.

•   You were not getting this error before the mod add-on?*
  

No I wasn't - infact Ive traced my steps back today and I havent modified any files today within zencart. I use the protx payment gateway - logged into my admin in Protx and turned on 3d secure check rules - thats all?!

Check the mod files and make sure you have included all the files.
I have - all fine. The error occurs when you click into the product description and then add to cart. If you add to cart fom the category listing with the 'buy Now' button its fine.

• You may also want to run the Debug utility found in the downloads section.*
  I 've tried this but Im not having much joy as there seems to be no log file generated. I've set the file permissions on /cache to write and still no report. Is there a switch in the admin I have o set to true somewhere?

Incidently Im running v1.3.7

Thanks Tina - any more help or guidance would be appreciated - Ive been hunting the forum and support for the last 3 hrs in a bid to find a solution but to no avail as yet (its big this forum), my boss is back from hols tomorrow and he'll be ringing me to "let me know".:shocking:
Thanks again
Nathan

Update:
Still no joy in getting the debugging script to work and a really long server error list where my customers are trying to add products which is giving them a 403 error?

Has anyone got any ideas or had a similar thing? Im seriously considering pulling the whole thing and doing a fresh install.

Thanks again in advance!

Further update:
It seems the error I was getting was server side and not anything I had done - phew! I contacted my provider and withing a few hours the issue was rectified!

I checked a few sites that are hosted with the same provider and all of the sites where kicking back the 403 error I described, I think it was an issue relating to this post:

http://www.zen-cart.com/forum/showthread.php?t=44762

But dont take my word for that - Im no expert!

Many thanks to Brinley at the welshshop for pointing me to this thread, and anyone who has thrown some words of advice at me.

Nath