mkdir permission denied

Hello! I enabled the D8 module with drush in order to get the libraries downloaded. However, whenever I visit a page with the slideshow, it doesn't cycle, and I get several of the following error messages in my log:

Warning: mkdir(): Permission denied in Drupal\Component\PhpStorage\FileStorage->createDirectory() (line 157 of /home/public/web/core/lib/Drupal/Component/PhpStorage/FileStorage.php) #0 /home/public/web/core/includes/bootstrap.inc(566): _drupal_error_handler_real(2, 'mkdir(): Permis...', '/home/public/we...', 157, Array) #1 [internal function]: _drupal_error_handler(2, 'mkdir(): Permis...', '/home/public/we...', 157, Array) #2 /home/public/web/core/lib/Drupal/Component/PhpStorage/FileStorage.php(157): mkdir('sites/interdepw...')

Since the paths are truncated, I can't tell what directory it's trying to make, but I assume it's in sites/[site name]/files. That directory is group- and world-writable, as are all the subdirectories. What am I missing? Thank you in advance.