Why is is "lacaj" here?
Why it is plural?
Because we're using a plural you.
Vi estos laca = you (singular) will be tired
Vi estos lacaj = you (plural) will be tired
That means the sentence is talking to more than one person.
Yet, "You will all be tired after that kind of work" was rejected.
Perhaps not all of the people being spoken to will be tired, just a group of them.
I would report it.
What's the difference between 'vi' and 'vi ĉiuj' in this context?