Hello Tobias

Unfortunately, it is quite common in the balkans to get no or no reliable informations in the railplanner. I recommend to check the pages of Serbian and Bulgarian State railways, and to be sure also write an email to them.

From Sofia, you can get to Dimitrovgrad by train:

https://www.bdz.bg/en/a/sofia-beograd-sofia

For the rest of the way to Belgrade, you need to take a bus (not included in Interrail).

Do not bother with these shuffles-only lengthens the time a lot and might even cost more.

I am aware that many young DE are very Prinzipenreiter-but be sensible and visit one of the dozens of small offices or countes in the Otogara beside or in fromt of Sofia station and bok a seat and bite the bullet to have to spend more money.

You mean book a bus ticket? I have no problem to buy a bus ticket as well. Seems like a little hassle to get around Serbia with the train. 

Yes book a busticket as Serbia have currently limited rail services :/

I encountered the same issue - Belgrade is a hub in the Balkans linking east and west and yet Belgrade - Sofia and Belgrade to Zagreb are not running due to COVID. I thankfully spotted it on Man in Seat 61. Train alternative is long winded via Hungary and Romania and very long.

Some more detail below about the proposed route and disruptions. Basically a bus is needed. Rome to Rio website was helpful for planning this.

https://www.seat61.com/trains-and-routes/belgrade-to-sofia-by-train.htm