What map are you trying to host? it may have a map protector thats preventing it from hosting because i've noticed that wc3 reforged can't host a lot of maps wc3 classic could. Pretty sure its because blizzard did something and it's conflicting with map protectors authors use to safeguard their work in certain cases. I saw your post up on official forums but couldn't respond because im silenced for swearing lol
Update: I found this other thread and after copying the files to documents/warcraft iii/maps as proposed, they can be hosted (even protected ones).
The first one I tried desynced/crashed the game though (showed score screen after maybe 60secs).