Can't run performance test (various websites)

I'm trying to run a Hummingbird performance test on a specific website, but receive the following error each time:

"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"

The same happens on some other websites we host, from time to time. There doesn't seem to be any pattern or commonality to the issue. It's only on a few sites, and has only happened in the last few weeks. Everything has been fairly smooth with running the performance test on all sites up until then.

Any suggestions would be appreciated. Thanks!