Problem handling Paypal IPN + PDT

[eluser]ecigraeme[/eluser]
Hi,

I am currently developing a site that uses paypal standard payments. When a payment is made paypal sends the information back to the site using an IPN notification and returns the user to the site along with the info as a PDT. I use both because I am giving digital downloads.

Both PDT and IPN verify that the information they have been sent really came from paypal by sending it back to paypal. If paypal recognises that it is a real transaction it returns verified.

Things all work fine in the sandbox but now that I have switched paypal to live it has stopped working.

When paypal forwards me back to the success page on my site. I see the following error, the IPN also fails with an INVALID response.

PHP Warning: require(../system/libraries/Exceptions.php) [<a href='function.require'>function.require</a>]: failed to open stream: No such file or directory in C:\Domains\mydomain.com\system\codeigniter\Common.php on line 116
PHP Warning: require(../system/libraries/Exceptions.php) [<a href='function.require'>function.require</a>]: failed to open stream: No such file or directory in C:\Domains\ mydomain.com\system\codeigniter\Common.php on line 116
PHP Fatal error: require() [<a href='function.require'>function.require</a>]: Failed opening required '../system/libraries/Exceptions.php' (include_path='.;C:\PHP5\pear') in C:\Domains\mydomain.com\system\codeigniter\Common.php on line 116

The strange thing is that when I refresh the page I don't get the error and the PDT information is verified and displayed as expected. I see the same error even with a die() statement on the first line of code in my success controller function, when refreshed the die() output is shown instead of the error.

I am using
$config['uri_protocol'] = "PATH_INFO";
$config['enable_query_strings'] = TRUE;

as suggested here to allow query strings and segments

I am using IIS and Pear appears to be installed.

I have the application folder at the same level as the system folder.

Does anybody have any ideas why this is happening.

Any help or insights would be much appreciated.

Thanks