Black screen when RDP over VPN

Situation: The client has a site-to-site VPN setup with two Sonicwall firewall. It works fine. He can ping anything on the remote sites and vice versa.

Problem: When he tries to RDP to the server he gets the blank screen and the banner at the top, but the log in window never appears and it eventually times out and says broken network connection.

Troubleshooting: Since this is DSL line, we suspect this is MTU issue. So, we use this commend "ping -l 1500 -f IP address" to test it. We receive "Packet needs to be fragmented but DF set" and "Packets: Sent = 4, Received = 0, Lost = 4 (100% loss)". That confirms that is MTU issues.

Resolution:  Use the ping to test and lower MTU. Finally, We reduce MTU from 1500 to 1400 in the SonicWall. Then he can use RDC over VPN.