There seems to be a bug in the process editor, when you use it to define an action that contains an argument with an underscore in the name. Once you save this action, which meets this criteria, you will not be able to open the action definition again through the process editor. You just get a generic error dialog when you try to open the action.

My action definition itself is minimal. It doesn’t contain anything other than the argument (screenshot after following recovery steps).

The underlying error that is found in the url is:

Error code: 0x80040216
Error description: Invalid variable name format

In order to recover from the error follow these steps

1.) Create a new solution and add the action to this solution
2.) Export the solution as an unmanaged solution
3.) Unzip the solution to a location
4.) In the workflow folder, you will find a xaml file. Open this using any text editor
5.) Find and replace the parameter name which has the underscore, to be without underscore
6.) Rezip and import

The action can be opened again after following these steps. I tested this issue and can confirm that it  happens in CRM 2013 6.1.1 and CRM Online.