PayPal API Internal Server Error

[scandalous]

I have been searching like crazy and can't seem to figure out this problem.

I am using the PayPal API and have everything setup correctly best to my knowledge.
I have a SSL cert setup and enabled SSL. The store runs in SSL (HTTPS).
I also configured the API settings with the username, password and signature.
Sandbox mode is off, express is off.
I have no image banner set.
paypalapi folder is set to 755 including all folders and files enclosed in paypalapi.

The problem I have is when going to the payment step and clicking on Paypal (red logo) I get the following:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

As I said I searched and can not find a solution. The normal Paypal (non API) works just not the API.

Any advice?

Thanks.

[scandalous]

Still no luck
What's weird is I have PrestaShop set up for a diffrent site on the same server with the same settings and file settings and that works.
This one just doesn't want to.

[rocky]

That error usually means a permissions error. Have you compared the permissions of the modules directory, modules/paypalapi directory and the paypalapi files on the non-working site with those of the working site?