I've taken your advise, and done some testing in quickfile_paypal_ipn(). I gathered info into a variable in the while loop starting at line 1057, and at the end I called the watchdog to store this variable into drupal's log. It shows the content in $res, the separator is "|".

HTTP/1.0 500 Error encountered | Connection: close | Content-Type: text/html | | | | | | |
Your browser sent a request this gateway didn't understand.
| |
Possible reasons:
|
|
Your browser violates the HTTP/1.1 protocol |
There's an interoperability problem between your browser and this proxy |
|
Possible solutions:
|
|
Contact your system administrator for assistance |
Contact your Zorp support for assistance |
|
Additional information:
| No 'Host:' header in request, and policy requires this. |
--------------------------------------------------------------------------------
| Page generated by Zorp on bendeguz, version 3.0.8 on Tue Feb 13 22:38:05 CET 2007. | | | |


Can you interpret this for me? Is it an issue for Drupal, for Quickfile or for my host?

Thx,
Doka