I've had this happen and never found a good answer. I had to restart my drawing from scratch, I'm sad to say. I suspect it had something to do with sheet set manager, but couldn't verify.
If you feel that this is related to River Analysis, please submit the feedback to the labs.civil3d.river email address.
I have seen others during my AU lab get the same error, but I have not been able to reproduce the situation that produces the issue.
I would suggest opening a support request. In the request, we will likely need to know what other Autodesk software is installed to determine what is causing the conflict and error message.
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