GZIP Text/HTML error

Hey,

When I want to GZP compress this website it gives me the following error code:

gzip is not working properly:
- Your server may not have the "deflate" module enabled (mod_deflate for Apache, ngx_http_gzip_module for NGINX)
- Another plugin may be interfering with the configuration

I've contacted our IT support and the deflate module is enabled. I've also looked at the installed plugins to find the interference and found nothing.