Typically when you have this issue, Workflow will write the C# to the logged-in user's temp directory. For the resource library, it'll generally be a largish .tmp file :) Securing the .tmp file and getting it to support is the best way to get it solved quickly.
If you're handy with Visual Studio, you can actually get to the bottom of the issues yourself. It involves opening a new C# class library, adding references to Core/Framework/generated resource library, and renaming the .tmp file to a .cs file and adding it to your project. Attempt to compile, and you should duplicate the errors you see in the Workflow GUI. But with Visual Studio, you can jump to the line quickly and see what the problem is.
(Fair disclosure) This is how I used to solve/troubleshoot the issues. Sometimes the fix required fixes to the template. Other times, we had to specifically block ceratin resources because Workflow can't support the type of inputs/outputs defined.