Why is Eurail not accepted on the trains between Novi Sad - Belgrade?
They are operated by Serbian Rail and they are part of Eurail.
I had troubles to find a empty seat as this route is the first Highspeed track in serbia and currently every serbian takes a test ride on this route.
i was told the trains are often sold out

@seewulf: I guess-there was a similar report lately (describing on how to get Bgr→ Bp on rails amap) that said same. I guess the seats by now are all REServed and they cannot yet? cope with RES only without the seat paid same time. Or they need the money.

@Hall: you are right, have found it and in fact we as helpers here have also stated exactly same many times here-and also the negative cons of passes in some other notorious countries. Even if there were trains-often they are so cheap that passdays cost more as simply buying there&then.

Though there are a few sneeky ways to cut the time in bus-and go over rails for die-hard train fanatics. Best site for current situation is seat61.com- and its very much pro-train

Is this route now with mandatory reservations? If so, was it a problem to get a reservation only?

There are IC trains where a reservation is needed. They stop just at Novi Beograd. 

There are (fast) regional trains where a reservation is needed. 

There are regional trains where a reservation is not needed and not possible. 

If you’d look at the trains you’d say: There’s a difference? :-)

The price in the last month was just 2,55 € with reservation fort the IC trains - wonder what a price without reservation would be but for this price I wouldn’t even argue about it when buying it because of this. Interesting thing is if a pass wouldn’t be accepted on the slow trains.

Any new updates on this? I am curious how the situation is at the moment. Is it actually even worth it to get an Interrail pass when traveling to and from these countries? 

According to the network guide in the app and the Serbia page, the Soko trains can now be used, with a reservation (around €1).