The two causes are RDP plug-ins and UDP usage under certain conditions.
Solution for RDP plug-ins
Go in the Experience tab of your RDP sessions and set Load plug-ins in embedded mode to No. This solution has a low impact.
Solution for UDP usage
There's an option in RDM to disable UDP usage.
- Open Remote Desktop Manager as an administrator.
- Click on Tools – More Tools – Local RDP/RemoteApp Manager.
- Click on Disable to disable the UDP Settings.
The research in this solution hint that this is caused only when going through a VPN, others mention that it occurs only on Windows 8 paired with a Gateway server, but it has worked for many of our users.
It requires a registry change, so all the usual warnings and caveats apply here, back it up first! Consult this Microsoft support article for more information.
The fix is to disable UDP on the client workstation:
- Navigate to HKLM\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services\Client.
- Create a DWORD named fClientDisableUDP and assign it a value of 1.