PDF Services API IO_SCHEMA_MISMATCH

Hello, I am trying to use the PDF Services API in a .NET 5 web project to convert DOCX files to PDF. I have tried the code in a test project and it worked there, but when I put it in the project where I need it, I receive a ServiceAPIException with the ErrorCode: IO_SCHEMA_MISMATCH and the following error.

Error response received for the request: Invalid I/O in the request. I/O name: $type I/O value: "Adobe.PDFServicesSDK.core.cpf.dto.request.platform.CPFOutputs, Adobe.PDFServicesSDK" null

The C# code is exactly the same between my test project and the actual, and I am even testing with the same Word Doc to try and narrow down the issue. The test file works perfectly in the test project, but not in the actual.

What is this error? Where does it come from? What is the solution?

Thanks