I've tried the usually suggested method of copying minecraft world from "saves" folder in ".minecraft" but Bukkit didn't recognize the world was even there. The world works. I've tested it a little in single player (the reason being it is 30k x 30k in map size). I'm not sure what I'm doing wrong (of course). I created the world from World Painter 2.4.1 and exported to Minecraft with unedited world (will paint it later when I know it works). I'm running and playing the server from localhost with default port. I ran and played the server with default world several times so I know the server works as it should. I'm playing Minecraft 1.12.2 (to let you all know I'm playing new version of Minecraft). I'm running server with 4 plugins, "Multiverse-Core" and the three Multiverse portal plugins.
With all this here. I've noticed the default world had "uid.dat" and mine didn't. If this is related to my issue, may someone please shed some light on it.
Best Answer
The world folder has to be called "world".
In general, if you are in such a situation, you can start with the default world, then copy the parts of the world one by one and see if they work. That way you can narrow down the cause.