It looks like the train may run, but leave at 17:49. Actually, it's only 1 sleeping car which is attached to the EC Prague - Linz and then shunted onto the night train to Zurich. The timings for the night train however don't seem to have been updated yet (showing departure in Prague at 18:01 instead of 17:49) and that's probably why it can't be booked. I'd suggest to try every now and then. It should be bookable via cd.cz.

Here a manual how to reserve the train on Cd.cz (usually the reservation opens 60days in advance in case of construction works it can be less )

The last sleeping car from Praha to Zürich is running june 30th. Next one available will be august 21st. 

So there's no way to book a bed because the sleeping car is simply not running. You may forget all booking ideas given because of that.

If you need to go, you may head to Wien or Linz from Praha and get a bed in a connecting train (if there's availability) from there. 

Kind regards, Hektor.  

Hm, so CD/ÖBB still haven't updated their timetables correctly :(

That's true indeed. But I got a letter from České dráhy confirming this. 

There is track construction on both sides of the frontier in july and august.  

Thanks for your answer and explanation!

Frustrating that we can't rely on the train planner on the interrail website.

We changed our plans and our next stop after Prague will be Bratislava on August 7. According to the train planner on the CD website it is possible to make seatreservations, but there's also a message about a lot of construction work on this connection. The details are only in Czech. Do you maybe know if we could just take this train? (for example: direct train, departure 10:56)

Unfortunately, that's the case. The national or operators’ planners are more reliable. For international planning, the DB website or DB Navigator app is a good choice. But all planners still depend on the data provided by the individual operators...

I just use DeepL or Google Translate :)

There are engineering works and the timetable has changed a bit. The timetable shown is already the adapted one. Reservation is possible and optional.