Question: We're on civicrm 4.7.5 on Drupal 7.

All contributions paid through the paypal standard payment processor are failing to successfully complete.

The user sees a successful transaction, but the contribution status is " Pending (Incomplete Transaction)"

There is nothing in the CiviCRM log to suggest an error. The IPN is sent to the correct URL, the status recorded in paypal is HTTP 200 OK.

But when I copy the path into a browser:

<https://[ourwebsiteurlredacted.com]/sites/all/modules/civicrm/extern/ipn.php?reset=1&contactID=9994&contributionID=5669&module=contribute&membershipID=9587>

this prints the following message to screen:

```
Sorry, can not load CMS user account.

```

Any help debugging would be greatfully received!

Answer: 
I think there is a bug here of some sort - I have reverted to an old copy of ipn.php, from 4.7.3, and payments are now completing successfully again
We had identical symptoms, after lots of cursing CiviCRM, finally found it was nothing to do with CiviCRM. 
PayPal IPN requires a **valid** email address. Our mailbox was full with someones vast supply of Spam. Cleared the mailbox and it worked fine.
Hope this does it for you.