Error 500 when adding a new signature on an "In progress" transaction

Hi. I have just started working with the API and the .NET SDK, because I wasn't able to add another signer from the sandbox WEB UI to a recently created transaction. This just occurs when one or more of the recipients have already signed the document. I tried to achieve this using the .NET SDK (you can find my code snippet attached) and even with the API directly, but I get an HTTP Internal server error. I wonder if this is really impossible, as I don't know the workflow of the system, or if there's another way to achieve this. I would really appreciate any help. Thanks!