Fix RDP An Internal Error Has Occurred

Problem overview

When initiating a Remote Desktop connection, the client abruptly drops and displays “An internal error has occurred.” The error prevents connection completely, often without providing an error code or specific reason.

Common causes

• Network Level Authentication (NLA) mismatch between host and client
• Blocked RDP port 3389 on the host firewall
• Remote Desktop Services failing on the target machine
• Corrupted cached credentials on the client machine

Solution 1: Check Network Level Authentication

1. On the host computer (the one you are connecting to), press Win + R, type sysdm.cpl, and press Enter.
2. Navigate to the Remote tab.
3. Uncheck the box that says Allow connections only from computers running Remote Desktop with Network Level Authentication.
4. Click Apply and try connecting again from the client machine.

Solution 2: Flush saved credentials

1. On the client computer, open the Remote Desktop Connection app.
2. Click the dropdown arrow to reveal options.
3. If there is a saved credential for the target computer, click delete or edit.
4. Alternatively, open Credential Manager, go to Windows Credentials, and remove any entries related to TERMSRV.
5. Reconnect and enter the username and password manually.

Solution 3: Verify Remote Desktop Services are running

1. On the host computer, press Win + R, type services.msc, and press Enter.
2. Locate the Remote Desktop Services service.
3. Ensure its status is Running and the startup type is Manual or Automatic.
4. If stopped, right-click and select Start.

Solution 4: Change RDP port or check firewall rules

1. Open Windows Defender Firewall on the host machine.
2. Go to Advanced settings > Inbound Rules.
3. Verify the Remote Desktop - User Mode (TCP-In) rule is enabled and allows traffic on port 3389.
4. If port 3389 is blocked by your ISP or router, change the listening port using the registry at HKLM\System\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp\PortNumber.