Blank page on upgrade from 2.1.4 to 2.2.x

set the error logs to level 4 and that informed me that I had not set the datetime element in my php.ini file (unrelated). It also gave me: Fatal error: Call to undefined function hash_hmac() in .../system/libraries/Session.php on line 162

Turns out I needed to install php56-hash-5.6.7 in my php installation. After installing the package and restarting apache all was well. Props @gadelat for getting me to the right direction.