Starting with 1.18.8, VS uses newer version of .NET Framework. Sadly, now the "native" server files (downloaded from vintagestory.at) are built specifically for x64 systems, and are incompatible with arm64 (Raspberry and other bigger ARM machines). Even if you switch from mono to dotnet 7.0 - whether it is installed manually or comes from Docker images - it won't be able to run the VintagestoryServer.dll, emitting the System.IO.FileLoadException error that OP mentioned.
There is now an experimental version of the ARM server from one of the Anego developers at https://github.com/anegostudios/VintagestoryServerArm64 . I built the Docker image from there and ran the 1.19.8 server on my Pi 5; at the very least it starts properly and connections work. Hopefully the amazing @Th3Dilli won't abandon this endeavor, and once 1.20 drops we will get the updated binaries for it too
The repository mentions that the discussion of the arm64 server is carried out in Vintage Story Discord #multiplayer -> VintagestoryServer Arm64 thread.
