It happens as soon as “Enable merging of CSS files” is enabled (even in 2.1.4 - no idea why it worked on your end). This is because WP-Optimize doesn’t convert the relative path correctly (which we haven’t changed recently), so that it ends up looking like this:
An alternative solution would be to exclude the following path from processing (see screenshot): /wp-content/themes/bricks/assets/css/libs/*