After installing the mod, and playing it, along with other mods, I decided that because the base metals mod interferes with BOP's world generation, I wanted to teperarily remove the mod to generate another world. however, upon doing so, loading, or generating a world will crash the game giving the unexpected error:

The game crashed whilst unexpected error
Error: java.lang.RuntimeException: World MpServer (net.minecraft.world.WorldProviderSurface) does not have BetweenlandsWorldStorage capability

-Or-

The game crashed whilst unexpected error
Error: java.lang.NoClassDefFoundError: thebetweenlands/client/ClientHooks

Even when removing the betweenlands mod, all other mods, and or even using a fresh forge&liteloader game version, the game will still refuse to load worlds, giving the same errors.

When you are removing all mods are you deleting the mods/ directory, or is it at least truly empty?

I emptied everything out of the folder except for sub folders that I put older version mods into.

Edit: turns out the mod creates a sub folder for the mod, and I confused it for an old folder. The problem stopped once I deleted it.