System.TypeLoadException: get_TargetFramework doesn't have an implementation

I'm trying to test some code using Nunit and as I try to debug a Test I get this message:

2019.04.09 14:56:12.490   ERROR System.TypeLoadException: Il metodo 'get_TargetFramework' del tipo 'NUnit.Engine.Extensibility.ExtensionNode' dell'assembly 'nunit.engine, Version=3.6.1.0, Culture=neutral, PublicKeyToken=2638cd05610744eb' non ha un'implementazione.
System.TypeLoadException: Il metodo 'get_TargetFramework' del tipo 'NUnit.Engine.Extensibility.ExtensionNode' dell'assembly 'nunit.engine, Version=3.6.1.0, Culture=neutral, PublicKeyToken=2638cd05610744eb' non ha un'implementazione.
   in NUnit.Engine.Services.ExtensionService..ctor()
   in NUnit.Engine.TestEngine.Initialize()
   in NUnit.Engine.TestEngine.GetRunner(TestPackage package)
   in JetBrains.ReSharper.UnitTestRunner.nUnit30.BuiltInNUnitRunner.<>c__DisplayClass6_0.<RunTests>b__0()
   in JetBrains.ReSharper.UnitTestRunner.nUnit30.BuiltInNUnitRunner.WithExtensiveErrorHandling(IRemoteTaskServer server, Action action)

Some tests just pass without any problems and others fail throwing this exception