I just had a similar problem. I tried a few solutions all at once, so maybe one of these will help you.
2. I received a notice from PayPal saying that the IPN was failing. I have Akeeba's Admin Tools component running so I added .paypal.com to the whitelisted domains.
3. I am integrating with Community Builder. I used its tools to check the database and found a problem with the user database. I followed the directions to fix it.
I ran a test with PayPal in test mode and it seems to work now. I will post again if I have further problems.