[NOT SOLVED] Server Error when trying to access pdf.php/ pdf-invoice.php

[Twinduct]

Hi Guys,
Running into walls here. Having this issue where I get a server error 500, everytime I want to view any invoice (back office + front).

I have set the premissions as follows

/Prestashop/ 755
/Prestashop/pdf-invoice.php 644 (Also tried on 755)
/Prestashop/Admin/ 755
/Prestashop/Admin/pdf.php 644 (Also tried on 755)

I get a 500 error when viewing the page as follows :
http://www.store.url/prestashop/pdf-invoice.php?id_order=8

But If I remove the id_order=8 .. or change it I get the following:
invoice not found


I've got the Apache logs (As I've read this might help to resolve 500 errors), but have no idea how to use these to my benefit. Looked up on Apache logs, but that didn't really make it clearer.

Could someone please help me with this? We're running a special for Valentines and getting alot of irate customers.

[Twinduct]

Ok have gone through the whole prestashop directory. All Folders = 755 and All Files = 644.
Also checked my .htaccess .. nothing weird in there, hell it just points to the directoryindex.