When caves are enabled it says "Dedicated server failed to start". Another strange thing that happenes is if i make a world without caves then leave the world then rejoin it, It then throws that same message and i cant open the world again. My friend has the same issue. This happened seamingly after our computers blocked file access for some unknown reason but we allowed the access and now this is happening. And yes this is without mods enabled.
Enable caves and generate world. And generate world without caves then leave (without picking character) then load world again.