Problem updating pipeline extension to MG 3.8

I found a content pipeline extension that I’d like to use with some modifications. As part of these modifications I am trying to update it to .NET Core 3.1 and Monogame 3.8 and I am having some problems.

The project compiles, but when building content I get the error error: Failed to create importer 'FieldFontImporter'. I have updated the reference in my .mgcb file. I get the same error if I completely remove the reference or point it at a non-existant file. Building content works if I try referencing a version of the DLL from before I tried updating the .NET and Monogame versions.

Thinking that the problem might be with the references to non-Monogame libraries, I tried removing those references completely and replacing their use with hardcoded values. It did not help.

My project file looks like this and the code is as the linked repo, other than the uses of external libraries replaced with hardcoded values.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <ApplicationIcon />
    <OutputType>Library</OutputType>
    <StartupObject />
  </PropertyGroup>

  <ItemGroup>
		<PackageReference Include="MonoGame.Framework.Content.Pipeline" Version="3.8.0.1641" >
			<PrivateAssets>All</PrivateAssets>
		</PackageReference>
		<PackageReference Include="MonoGame.Framework.WindowsDX" Version="3.8.0.1641" >
			<PrivateAssets>All</PrivateAssets>
		</PackageReference>
  </ItemGroup>

</Project>

I tried googling the error and it doesn’t seem to have one specific cause and nothing I found really helped.

I noticed that the Monogame Pipeline Extension template targets netstandard2.0 and uses DesktopGL. I tried making these changes to my project, but it is still the same situation: The project builds with no errors, I update the reference path in the mgcb file and building content fails with error: Failed to create importer 'FieldFontImporter'.

UPDATE:
I discovered the cause of my this error. For some reason I was running an old version of the MGCB-Editor. I ran a dotnet tool update -g dotnet-mgcb-editor and I am now getting far more descriptive errors that I actually have a chance of solving.