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Errors while loading a map
For ex. 1.8 world to higher version (1.18+)
If you are trying to load a world from a lower version (ex. 1.8) to a higher version (ex. 1.18) and it isn't working, you have to convert that world by yourself. It's nothing hard.
The first step is to get the lower world into your singleplayer worlds (%appdata% -> .minecraft -> saves). After that, join to the world in singleplayer with the Minecraft version you want the world to have.
Like if you join there with 1.18, the world will be converted to 1.18. After all that you successfully converted it. Now get the world from saves and put it on your server.
Good to know: You can create a support ticket, describe your problem and, send proof of purchase so that you can be provided with the correct world download