I am having a similar problem with a library I develop. Dynamo shows a problem with dependency compatibility (protogeometry.dll i think).
So first thing - maybe you can check messages in the Dynamo console - maybe it is also a dependency issue.
Second thing maybe somebody could explain how to solve the dependency problem In my case I am using Dynamo 2.0.3 (on rvt 2019 still), but I am getting my references in Visual Studio through NuGet. Does this mean it is downloading for Dynamo 2.1 or smth? Should I add a local reference from my Dynamo install?
Sorry to slightly hijack the thread, but I think it might be a similar issue.
I can’t see anything in either the release notes, or in the properties of the DLL which indicate the .NET Framework version. But the release notes show the last version is quite old (from 2013) so guessing this is the problem & that the Dynamo primer needs to be updated with a different example