Have you tried deleting the C# project and solution, adding a C# script (can just be the default Godot-generated one), building the project, and then enabling it again? That is what I would try doing if you have not already, as the plugin was last updated for a version of Godot that was a few releases ago, so its C# generated files probably are out of date and need to be regenerated.
If that does not work, please let me know. If you could also include your OS (Windows, Mac, Linux), Godot version, and possibly the log file, that would be great as it would help me narrow down what the issue is.
Thanks!