[Hummingbird] Hummingbird "Expires Module" Error

We have a an error when trying to turn on browser caching:
Browser Caching is not working properly:

- Your server may not have the "expires" module enabled (mod_expires for Apache, ngx_http_headers_module for NGINX)
- Another plugin may be interfering with the configuration
However, I've confirmed that we have this module enabled...so here I am, to open a ticket. Hope you can help.