You can book it there but it will be a normal ticket with reservation.

A reservation only is only possible at a ticket office in Croatia.

Yes, im aware of that, but my question is if you know anything about if it runs on the night since alot of people say that it doesnt and randomly gets cancelled etc.

I haven't heard or read anything about that so I wonder who "a lot of people” is. And was that specifically for the domestic night train Zagreb - Split?

I'd expect any cancellations to appear here:

https://www.hzpp.hr/en/train-on-time-2?p=361&mp=545

Maybe watch that page for a few days?

AFAIK there used to be a train like that, but since this yr it has been lenghtened (distance a bit too short for real night) in Osijek-via Zag-to seasidesunny Split. And only ev 2nd day-though maybe in hi-season not. On nights with works on the line it may be rerouted and skip Zag.-or maybe stop at a suburban station youve never heard of and will not know how to get there.

Unexpected surprises-mostly negative-indeed do happen more often in those parts of the world as around here, but that does not say its frequent.

@rvdborgt  Iassume the Regiojet is mentioned as this train was often canceled between ogulin - Split and replaced by buses due the heavy delays :/

Possibly, but that train cannot be booked on the HZPP website. So I really wonder if the reports (by "a lot of people”) are for the domestic night train.