[Hummingbird] Can't clear cache

I'm getting this error when trying to clear the cache on a subsite: There was an error during the cache purge. Check folder permissions are 755 for /wp-content/wphb-cache or delete directory manually.
I've checked the permissions and they are 775. I'm using an amazon lightsail instance of a bitnami wordpress stack. The onwer/group of the wphb-cache directory is daemon:daemon. I changed the owner/group to bitnami:daemon but still have the same problem.