Alright, im able to reproduce, and this seems to be a bug in the Mojang Realms library.
They are *supposed* to only be accessing the Minecraft codebase through purely the deobfusicated API.
Due to us doing the deobfusication of Minecraft at install time. Those classes it's referencing does not exist at all.
We will look into fixing this, but it may require a fix on Mojang's end. We'll see.