I think the train is simply not open for booking yet. Maybe it only opens 2 months in advance, or maybe 3.

Deutsche Bahn should also be able to book it (ticket office or by phone: +49 30 2979), but of course only after it has been opened for booking.

I´ve read somewhere that this hitherto overnight train does not run anymore like it did-though the last yrs only in summer.

It will be changed into a longer trip Osijek (is in the east of Hrvatska, near HU and RS borders)-Zagreb (stops/passes like 2-3 am)-Split. And perhaps not daily buy every 2nd day or 3 t/week. Perhaps that date is for them too early to be summer-usually starts 1/7 over there. (check planner if it will run on later dates)

I assume it is by now still undecided what the plan will be, plus that it also seems large parts of the railline need to be upgraded and thats often done at-well, guess; Nighttime!