If you run across this error in Drupal 8:
Fatal error: Class Drupal\Core\Session\SessionHandler contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (SessionHandlerInterface::read)
Or something like it and nothing seems to get rid of it in most situations the best course of action is to restart your server. I don't think no one knows what exactly causes it yet but considering that a restart alone fixes it my immediate guess would be some sort of bug with the PHP Opcache.
I sometimes get this more often during development and I constantly run and shutdown XDebug. Perhaps that interferes with the Opcache.
Oh well.