I had the same problem as http://magento.stackexchange.com/questions/141252/user-login-not-work-after-supee-8788-patch

The suggested answers did not work for me, so I reversed the SUPEE-8788 Patch OK.

But still I cannot login to customer accounts.  Why would that be?  

I am using /template/persistent/customer/form/login.phtml as my login file.

I am also having an issue of not being able to login to admin after logging out. Again I have seen this linked to 8788 (php under 5.6 - I have 5.4).  But still have issue after revert.

what is going on?