Monolog cannot write logs after upgrading to PHP 7.0

Every request to the server results in this as it's response:

Error in exception handler:
    The stream or file "/home/forge/default/app/storage/logs/fpm-fcgi-2016-09-09.log" could not be opened:
        failed to open stream:
            Permission denied in /home/forge/default/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:107

I've checked, and the dir permissions are 0755.

Server system is Ubuntu 14.04. Nothing in the Nginx logs suggesting there is a problem there.

What might be going on?