We do know some things:
This type of error seems very rare and I believe it could only be raised of there was an internal networking problem on Let's Encrypt's side, where Boulder can't communicate with Unbound. Maybe a server in the pool died and you got unlucky.
That is distinct from a generic query timeout, where the blame may well be placed on an external nameserver.
I guess we can wait and see whether more people report the same. Unless Let's Encrypt want to chime in with whether they already know about it.