A lovely city. But not the friendliest one.
I travel extensively; 15-20 counties in the last 2 years. I'm friendly and very outgoing. But in my humble opinion, Belgrade (very generally speaking) is not a terrible friendly place for foreigners or solo-travelers.
It is a beautiful, inexpensive and very safe city. Prices are fantastic, as is the food. But unless you come here with a companion...be prepared to have a rather lonely time. The local Serbs are not rude or actively unpleasant, and most everyone speaks English well.
But most of the locals don't seem particularly open to having conversations with people they don't already know.
In most of my travels, simply by being a foreigner in a cafe, restaurant or bar...you will, at minimum, at least be able to strike up a conversation with, say, your bartender or waitress. Here in Belgrade...not so much. For example: if you're sitting at the bar in a pub...the bartender, given a choice, will usually prefer to read a book or stare at their phone, rather than conduct anything more than a brief, monosyllabic conversation. It's pretty much the same in shops, restaurants, etc. And if you're used to striking up friendly conversations with fellow patrons at a restaurant, or bar, or art exhibition...don't expect that in Belgrade.
Even the cashiers in the local grocery stores seem to prefer to keep interactions as brief as possible. Fake as it may be, there is something a bit comforting about the Western retail-facade of people pretending to be happy to see a customer...especially when you encounter the absolute lack of it for weeks on end.
I'm not saying it's good or bad; it's just the way things are. I suspect it's at least somewhat cultural. I've gotten somewhat similar vibes in a few places in the Balkans and Eastern Europe before. I would not rate the locals of Budapest or Kiev as overly-friendly to outsiders, for instance...yet I found it far, far easier to meet and talk to people in those cities than in Belgrade.
There are exceptions to the above. But I will stand by it as an accurate generalization, having been in the city for 3 weeks. Perhaps if you hook up with other travelers in some of the city's co-work-spaces, you might have a better time; I don't know.
PS: I also think the "Belgrade is famous for it's nightlife" reputation is severely over-rated. That reputation might have been appropriate in the past; I heard from a couple of locals that, as of a year or so ago, a lot of new laws were passed restricting the operating-hours of most bars and clubs in the city. Most (not all, but definitely) places in the downtown area close up by midnight or 1am at the latest.
With the exception of a couple of specific nightclubs...most of the city is pretty much a ghost town by midnight, even on the weekends.
Again...a beautiful city, with lots of interesting culture, art and architecture. I think I'd likely love it if I came with someone I was dating. But for a solo traveler...it's genuinely one of the least-friendly cities I've ever been to.