Can't enable Minification

We can't enable the Minification. Once Hummingbird finishes checking the files, we are redirected back to the "get started" panel.
Things already done:
- Plugin and theme conflict test already made;
- No relevant errors in debug.log;
- I contacted the host provider and they mentioned that Varnish cache is disabled;
- Cloudflare was disabled during the tests (not sure if it's still disabled);