upgrading from 2.1.x to 2.2.x - 'cache path is not writable'

i am upgrading my test site from 2.1 to 2.2 (latest) and found that the cache is broken. the site ran fine on my local machine (installed fresh, not an upgrade). After performing the various necessary steps on my test server, i see this in the logs and the CSS/JS files don't load in the browser:

#6 [internal fu" while reading response header from upstream, client: xx.xx.xx.xx, server: test.ureka.org, request: "GET /cache/0/default/lightbox/elgg-colorbox-theme/colorbox.css HTTP/2.0", upstream: "fastcgi://unix:/socket/php-fpm.sock:", host: mysite.org", referrer: "https://mysite.org/admin/dashboard"
2017/03/28 09:48:16 [error] 30056#0: *490331 FastCGI sent in stderr: "PHP message: Exception at time 1490694496: Stash\Exception\InvalidArgumentException: Cache path is not writable. in /mysite/vendor/tedivm/stash/src/Stash/Utilities.php:231

i have checked all the steps i am aware of, but haven't resolved this yet.

the config file, database and data folder paths have not changed since the upgrade.

i tested this with and without the symlink for caching.

anyone know what i am missing?