[Hummingbird] Error 500 after activating Page Caching

Hi.

I tried the test a few times and got this message: "The performance test didn't return any results. This probably means you're on a local website (which we can't scan) or something went wrong trying to access WPMU DEV. Try again and if this error continues to appear please open a ticket with our support heroes".

When I turn on Page Caching I get error 500 when trying to access the Home Page.