Hello, thanks for the clarification.
We have tried to reproduce the issue this way and noticed that that you might be generating different initialization vector in the UWP app and in the server app, which would return different salt at the decrypting end. This would explain the error.
When we use the same IV/salt at the server side, the process works fine - download the updated sample solution and give it a try.
If you are doing anything differently, please let us know.