Internal Server Error 500 using PayPal Express

Hi,
I am quite new here. I have a webshop running OC 2.3.0.2. I am trying to add PayPal Express checkout. I enabled the plugin, added in the API keys, got a sandbox account for testing. All seems to be fine, up till the point when I get redirected back to my site from the paypal window and then instead of getting to the order confirmation page, I get an internal server error 500 error message. The OC log has totally nothing, the apache server log has nothing useful. This is a new installation, not an upgrade. I googled the problem, but found nothing useful.
If I add the express checkout button and use that, I am redirected properly back to the site, but then I get an OC error saying the shipping is missing, so like if the cart was broken. Obviously, as there is no confirmation of the order, it does not get into the database at all.
Could you please give me a hand where I should start the investigation and how I can resolve this problem?
Is it worth playing with this or should I just simply go with PayPal standard?
Thanks in advance
Zoltan

So Internal Service Error 500 (ISE 500) is a generic way that servers block the underlying error. This error is normally logged to a separate "error_log" within the root of your store. This file is a server-based file, unrelated to OpenCart or any other platform's own error log as it generally means the erro happened before the actual platform code could load its own error log.

To see the actual error, follow the steps here: http://forum.opencart.com/viewtopic.php ... 63afe56a01

Hello, i have installed the last version 3.02. But my problem is when try edit the payment of "Mercadopago". I click to pencil for edit and appears HTTP ERROR 500. im going crazy now