There have been two past threads about this error message:
It's hard to say what's wrong -- it's an extremely vague error message!
(But Let's Encrypt has better server-side logging now, so they can probably figure it out.)
It could be some sort of Let's Encrypt failure, and it's worth trying again.
But it could also be an ordinary validation failure -- for example, an Internet issue causing DNS resolution of or HTTP requests to your site to time out from one of the VAs -- except the error message got eaten.