When I add the latest nuget packages for Arch (2.1.0) and Arch.Persistence (2.0.0) I get the following error when ArchJsonSerializer is instantiated:
Exception has occurred: CLR/System.TypeLoadException
Exception thrown: 'System.TypeLoadException' in MonoECS.Core.dll: 'Could not load type 'Arch.Core.Utils.ComponentType' from assembly 'Arch, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null'.'
at Arch.Persistence.ArchJsonSerializer..ctor(IJsonFormatter[] custFormatters)
If I downgrade Arch to 2.0.0 it works as expected