It's a fairly straightforward question.
A creature has a Multiattack option, and it also can cast the Haste spell. While under the effect of Haste, can it use the second Action to take another Multiattack Action?
My main concern is the clause of the Haste spell that limits the number of attacks made from taking the Attack Action. This is a different Action, though.
Then there is the concern of being able to use Multiattack at all, because Haste says what kinds of Actions you can take.