Hello there,
This error is often caused by Windows policies that prevent incoming RDP connections, or simply your system's username.
This might be an issue with the permissions of the group that you have given to the users. By default members of the Administrators group have this right.
I would suggest you Verify RemoteApp and Desktop Connection Functionality by following the below link
Also have a quick recap on the Installing and Configuring RemoteApp https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-2008/dd772730%28v%3dws.10%29
The below thread discusses the same issue and you can try out some troubleshooting steps from this and see if that helps you to sort the Issue.
Hope this resolves your Query !!
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