This appears to be related to the latest Visual Studio update that has also updated MSBuild. In the MSBuild update it appears that System.Memory was updated (dotnet/msbuild#7680) which is possibly causing this issue as seen in some other projects:
microsoft/nodejstools#2407
unoplatform/uno#9297

I cloned the R4MVC repo and installed System.Memory 4.5.5 to the R4Mvc.Tools project and that appears to fix the issue?

Thanks a lot for the help @JackGilmore

Is it possible to publish the new version?

I'm not the project maintainer so I can't publish a new version to nuget. I'm happy to open a pull request with the required changes though. I'm not sure how likely it is that it'll get merged though as the last commit in the project was over a year ago and there are already pending pull requests 😐

I've opened a PR for this: #173

Dear friend
You can manually generate the nuget from the project and send it to me because I urgently need this nuget.

Hi @rbsdotnet. I don't have the time or knowledge to generate the nuget package but please feel free to generate it from my fork if you need to use it: https://github.com/JackGilmore/R4MVC