Can you restore inventories to a Minecraft server?

The import of Multiverse didn't work because I had to restart the map and change the server.properties.

Now I have the problem that Essentials has somehow recognized it as a "new world" and has simply cleared the inventory.

I don't know if it was exactly because of Essentials, but since you also got the Essentials starter kit, I think that was it.

Actually just wanted to ask whether it is possible that you can find a file in the world folder and thus possibly restore your inventory?

In Vanilla or Bukkit / Spigot there's a folder Playerdata in the world folder in which the data for each player is saved.

If you still have the original folder, you can copy the files out there.